I've seen people say otherwise, but Feyre's POV on Rhys isn’t at all unreliable. It's actually the opposite: she is by far the best POV we can get on him from a reader's perspective, other than Rhys's own.
Even discounting that Feyre has seen (and believed) the worst of Rhys so it's not like she's just been madly in love and thinking the best of him throughout her POV-
I stared at him, sending as much hate as I could into my gaze. He’d been the one who’d caused all this. He’d told Amarantha about Clare; he’d made Tamlin beg.
“Well?”
I bared my teeth. “Go. To. Hell.” -acotar
The reliability of her, and every non-omniscient narrator, is mostly rooted in how much they know and tell the reader.
And Feyre is the character who knows Rhys the best in the series. Feyre gets to see sides of Rhys that no one, not even his closest friends, do. She gets his thoughts and emotions through their mating bond. She relates to him and understands his motivations better. She spends more time with him, so we see more of his actions, words, thoughts, and feelings on page with her.
She is inarguably the best narrator on him because she's giving the reader the most information.
The king’s taunt to Rhys had been roiling through my mind for days now. Hybern expected him to give everything—everything—to stop them. Had claimed only that would give us a fighting shot. And I knew my mate. Perhaps better than I knew myself. I knew Rhys would spend all of himself, destroy himself, if it meant a chance at winning. At survival. -acowar
I knew why his eyes sometimes turned distant, why he occasionally just blinked at all of us as if not quite believing it and rubbed his chest as if to ease an ache. -Feyre, acofas
He’d always understood me best—more than the others. Save my mate. -Rhys, acofas
Bryce, Nesta, Ember, even Cassian, these other characters get pieces of Rhys and can (and do) draw incorrect conclusions about him as a result.
In an interview talking about seeing Rhys and Feyre from new POVs, sjm said of acosf:
"Rhys from Nesta's perspective, where she thinks he's like an arrogant douchebag and we know he's not."
Nesta thinks Rhys is arrogant, we see her think and get frustrated at that fact a lot, but from Feyre's POV, we see Rhys's vulnerabilities and doubts and learn about the facade of confidence that masks the truth:
I wondered, then, with his hands beneath my breasts and between my legs, what Rhys wouldn’t give of himself. Wondered if … if perhaps the arrogance and swagger … if they masked a male who perhaps thought he wasn’t worth very much at all. -Feyre, acomaf
Sjm says this because Nesta's perspective on Rhys is naturally limited because she doesn't understand him or really spend time with him. Nesta believes Rhys's arrogant mask because she doesn't know him well enough to doubt it, hasn't seen as many sides of him as Feyre, and isn't privy to his innermost thoughts.
It's also why she says stuff about Rhys that we know is untrue:
Rhys said tightly, “I don’t wish to be High King. I only wish to be here, with my mate and my people... I will not be High King. I will not consider it, not today and not in a century.” -acosf
“And Rhysand is … your king?”
Nesta snorted. “He’d like to be.” -CC3
That's not to say Nesta or Cassian or whoever are unreliable narrators (none of the acotar narrators fit that by definition), but readers get less information about Rhys's actions and feelings and motivations from their POVs, so you have to guess at some aspects or use knowledge we have from Feyre's (or Rhys's) POV to understand.
Like how sjm was counting on readers would read Nesta's POV where she talks about how Rhys is so arrogant and know better because we'd seen moments like this before that prove thinks more lowly of himself than he seems:
“Perhaps I don’t know what I want, but at least I don’t hide what I am behind a mask,” I seethed. “At least I let them see who I am, broken bits and all. Yes—it’s to save your people. But what about the other masks, Rhys? What about letting your friends see your real face? But maybe it’s easier not to. Because what if you did let someone in? And what if they saw everything, and still walked away? Who could blame them—who would want to bother with that sort of mess?”
He flinched.
The most powerful High Lord in history flinched. And I knew I’d hit hard—and deep.
“The other night you told me you wanted a distraction, you wanted fun. Not a mating bond. And not to someone like me—a mess.” So the words I’d spat after the Court of Nightmares had haunted him. -acomaf
And Cassian knows Rhys a lot better naturally, but we know he doesn't get as much from Rhys. In acomaf and acowar, it's repeatedly shown and discussed how much Rhys tries to hide vulnerabilities from even the IC. It's why Feyre says above, "what about letting your friends see your real face?", because Rhys won't let Cassian and others see the full picture, going out of his way to do things like hiding his nightmares from them in acomaf. And while they're often aware something is wrong, Rhys is known to pretend with them.
Cassian even remarks on this:
Cassian had witnessed Rhys going deep into his own head often enough. Knew his brother was prone to withdrawing while appearing perfectly fine. -acosf
But we know, even before they're mated, that Rhys confides to Feyre things he won't tell the IC, like on Starfall:
“Every year that I was Under the Mountain and Starfall came around, Amarantha made sure that I … serviced her. The entire night. Starfall is no secret, even to outsiders—even the Court of Nightmares crawls out of the Hewn City to look up at the sky. So she knew … She knew what it meant to me.”
I stopped hearing the celebrations around us. “I’m sorry.” It was all I could offer.
“I got through it by reminding myself that my friends were safe; that Velaris was safe. Nothing else mattered, so long as I had that. She could use my body however she wanted. I didn’t care.”
“So why aren’t you down there with them?” I asked, even as I tucked the horror of what had been done to him into my heart.
“They don’t know—what she did to me on Starfall. I don’t want it to ruin their night.” -acomaf
There's even moments in acowar where Rhys insists something is true to the IC and then admits otherwise to Feyre:
“If Amarantha showed up at that door right now,” Rhys snarled, pointing toward the foyer entry, “and said she could buy us a chance at defeating Hybern, at keeping all of you alive, I would thank the fucking Cauldron.”
Mor shook her head, tears slipping free again. “You don’t mean that.”
“I do.”
Rhys murmured, “If she …” His swallow was audible. “If she showed up at this house …” I knew who he meant. “I would kill her. Without even letting her speak. I would kill her.”
“I know.” I would, too.
Feyre's narrative is always going to be the best for the reader to understand Rhys other than Rhys's own POV because we get so much more of Rhys from her than we will get from someone else.
Another narrator won't let us feel Rhys's feelings, know the parts of himself that he hides from even his closest friends, get his unfiltered thoughts, and even just see him more because Feyre and Rhys spend so much time together.
Because that's another thing, Rhys isn't seen as much in non-Feyre POVs because they spend so much time with each other. That means we have to fill in certain gaps. We have to extrapolate from what we do see. For example, in acosf, Rhys tells Cassian he's having nightmares about Feyre dying and has a panic attack in front of him and shows impaired critical thinking in one or two scenes:
That Rhys couldn’t understand what Az meant told Cassian how distraught and terrified he was.
But most of Rhys's reactions are not shown to us beyond that; Rhys actually seems pretty composed most of the time with just the mildest indications of what we know: that he's a mess in a lot of acosf. We don't even know tons of details of how he learned about the baby having wings or what exactly is said, his immediate reaction.
Rhys panicking in front of Cassian is after he's spent several scenes with Cassian, knowing and freaking out, and Cassian only notices something's off because his smile doesn't reach his eyes and he seems distracted. Cassian was clueless how much of a wreck Rhys was until he confesses to him because of how good Rhys is with pretending he's fine and wearing a mask.
So we just have to make the intuitive leaps on what's going on with Rhys behind the scenes, whether his terror making him miss what should be obvious strategic connections in one scene means there are other things he's missing or other ways he's acting uncharacteristically.
There's just a lot that we don't know about Rhys in acosf because we just see him less and get told things rather than shown it as was more common in Feyre's POV.
Feyre also loving Rhys doesn't negate that she's also giving us the most information for us to judge his character, to actually understand him, to see beneath the surface of the masks he wears.
Through the bond in my hand, I could have sworn I felt a glimmer of pleased surprise. I checked my mental shields—but they were intact. And Rhysand’s calm face revealed no hint of its origin. -acomaf
I could have sworn I felt Rhys flinch through the bond. But my mate said calmly, “We did nothing. Hybern chooses its actions, not us.” -acowar
Feyre is constantly telling us in her POV that Rhys is externalizing something that contradicts how he really feels. Even beyond him confiding in her things he won't even tell his closest friends and showing sides of himself he won't show anyone else, Feyre understands Rhys better because she literally feels his emotions. This began in acotar, too:
“Your court fell, too.”
Sadness flickered in those violet eyes. I wouldn’t have noticed it had I not … felt it—deep inside me. My gaze drifted to the eye etched in my palm. What manner of tattoo, exactly, had he given me?
And then she is also constantly relating to him, she understands him what motivates him and why he makes certain choices not because she loves him, but because she's like him and she recognizes this before she's even aware they're mates:
Rhys and I were one in the same—beyond the power that he’d given me. -acomaf
Feyre has a lot of similarities with Rhys, which is commented on by others:
He gave me a grim smile. “You can rely on us, you know. Both of you. He’s inclined to do everything himself—to give everything of himself. He can’t stand to let anyone else offer up anything.” That smile faded. “Neither can you.” -Cassian to Feyre, acowar
It's part of why she forgives choices he makes that she gets mad about, because she's able to see where he's coming from and just understands him:
But he'd known I'd react badly. That it'd hurt me more than help me.
And what if I had known?
What if I had known that Rhys was my mate while I’d loved Tamlin?
It didn’t excuse his not telling me. Didn’t excuse the recent weeks, when I’d hated myself so much for wanting him so badly—when he should have told me. But … I understood. -acomaf
Rhys not sharing burdens with his family is a trait he shares with Feyre and one they acknowledge:
“They’d be happy if you let them shoulder the burden.”
“The same way you rely on others to help with your own troubles?” -Feyre telling Rhys to confide in the IC the way he does for her, acomaf
She and Rhys have tons of parallels and similarities, but they also just relate because they understand (and even share) each other's trauma:
"I felt your pain, and sadness, and loneliness. I felt you struggling to escape the darkness of Amarantha the same way I was." -Rhys to Feyre, acomaf
Maybe he only understood because he, too, had been helpless and without choices, had been forced to do such horrible things, and locked up. -Feyre on Rhys understanding her trauma, acomaf
This was mentioned multiple times by sjm, too:
"Rhys has his own history that allows him to have some perspective on how Feyre might be feeling." -on Rhys being respectful and treating Feyre as an equal
"He and Feyre are both battling trauma, despair, anxiety, and tremendous guilt. And their journey from that dark place to learning that they are worthy of being loved and accepting love is so important to me." -on acomaf's healing journey
While none of this means people can't analyze Rhys outside of Feyre's POV, everything we see of Rhys inside Feyre's POV is intended context to understand Rhys's actions and motivations outside of her POV, when we get less information on him and have to extrapolate more.
That she loves him and thinks highly of him later doesn't change any of that, especially because we have tons of examples of readers liking characters that all the narrators dislike or being critical of characters who the narrator loves. Outside of if someone projects onto narrators that their emotions become yours, Feyre's feelings shouldn't really bias readers much, or obscure any of the information that can let everyone draw their own conclusions.
But because Feyre's POV on Rhys is the most complete, detailed, and informative one we have, the one that lets us really understand why and how and what Rhys does and thinks and feels, it is also the most reliable one.
