It's really interesting too how both Percy and Nico seem to have beheld the full horror of Tartarus whereas Annabeth (I would argue) never did.
When you read the first few chapters, P & A are actually BOTH coming to the realization that the place itself is alive/is the body of Tartarus and they never talk about it because they don't want to scare the other one, like Annabeth is hoping Percy hasn't noticed this disturbing detail and vice versa.
But then we get this narration from Percy:
Percy realized that what he saw of Tartarus was only a watered-down version of its true horror - only what his demigod brain could handle. The worst of it was veiled, the same way the Mist veiled monsters from mortal sight. Now as Percy died he began to see the truth. The air was the breath of Tartarus. All these monsters were just blood cells circulating through his body. Everything Percy saw was a dream in the mind of the dark god of the pit. This must have been the way Nico had seen Tartarus, and it had almost destroyed his sanity.
Annabeth never has a moment like this and this revelation explicitly links Nico and Percy's experiences - somewhat telling, because I think they are both much more visibly messed up and have a much harder time recovering than Annabeth does. Yes, Annabeth is clearly shown to be struggling in BOO, but not to the same degree as the boys - Percy is narrating that "Nico’s eyes looked like shattered glass. Percy wondered sadly if something inside him had broken permanently." Interesting connection to the Ahklys scene where Percy "concentrated so hard that something inside him cracked - as if a crystal ball had shattered in his stomach." (Could be a coincidence in language choice. Probably is. But the 'shattering' language is significant all on its own!)
Then in BOO Nico's rage is exploding and he's putting himself in a coma from unleashing all his pent up emotions. And in Piper and Jason's perspective you get the sense that Percy is like quite literally on the edge of a complete breakdown in BOO. IMO Annabeth's struggles read a lot more like run-of-the-mill PTSD and feelings of helplessness, not that she's on the edge of insanity or considering harming herself.
I also think it is noteworthy that Percy has that revelation before his encounter with Ahklys. And that even in the encounter with her the reason he is actually able to control the poison is because 1. something inside him breaks 2. he realizes that, "Tartarus had its own rules. Fire was drinkable. The ground was the body of a dark god. The air was acid, and demigods could be turned into smoky corpses."
Like he is quite literally using the knowledge he acquired of just how much of this place is an illusion in order to bend the laws of the natural universe. It's almost like he hacks the system, becomes self-aware within the simulation. He realizes the only thing holding him back is his understanding of his own limitations, but the 'rules' are different in this place. It seems to me like this breakthrough is made possible directly BECAUSE of his revelation as he is dying.
After this point too he does his INSANE like 500-yard leap from the Mansion of Night (which Annabeth literally cannot fathom how he was able to do!) and I think (we're never told how he did it) that he was using the same logic that he uses in the Ahklys fight and his powers responded to it. And before this revelation he also can't control the rivers in Tartarus, and after it he can and does with ease.
Anyway! I think the psychological horror element to Tartarus is its strongest and most pervasive weapon and Percy is substantially and significantly worse off than Annabeth because he saw this horror unmasked
