"Ao3 is perfect! I love Ao3! Everyone should donate to and support Ao3 if they care about fandom as a whole!"
Ao3 is dogshit and I hate it because:
Ao3 is pro Degradation, Violence, Abuse, and Abuse Simulation (aka., pro DVAAS) because they are pro these subcategories of DVAAS;
Pro written and drawn Child Sexual Exploitation Material (aka., CSEM) [ie., sexual material of real life children and teenagers, which is inherently exploitative as children and teenagers cannot truly consent to, let alone ethically and truly approve of or partake in the creation and distribution of sexual material depicting them and their underaged likeness]
Pro pedophile/rapephile in general [there is a literal "pro minor attracted people" collection on their website that they support and defend the right of to exist on their platform, plus hundreds upon hundred of fics featuring eroticized depictions of pedophilia, zoophilia/bestiality, necrophilia, incest, etc.]
Pro Fictional Child Sexual Material (aka., FCSM) [ie., eroticized material of child and teenage characters, of which is harmful because it desensitize normal people to pedophilia making them less likely to care about victimized children and or to take the suffering of victimized children seriously, while also normalizing pedophilia to pedophiles increasing the likelihood of them go on to then victimizing a real child]
Pro rape culture and abuse apologia in general [eg., support and defends "ship fics" involving adult/minor relationships portrayed as good, sweet, and loving, or involving the portrayal of the minor as the instigator or predator and the adult as the helpless one who just can't resist or even as the victim being abused, along with "dub-con" as a concept being allowed as something separate from "full-blown" or "outright"Â "non-con"/sexual abuse when "dub-con" doesn't exist. Sexual acts are either explicitly consensual or it's sexual assault, no excuses or exceptions.]
"But Ao3 is just an archive so they have to allow everything that's legal or else they wouldn't be an archive! It being allowed doesn't mean that it's an endorsement!"
Archives exist to persevere information and or objects (which Ao3 fails at by allowing people to fully remove their works form their site), and they don't have social futures (kudos, bookmarks, and comments) that can be use as way to indicate a quality of a work (more kudos means more people liked it, less kudos means less people liked it, hence, indicative of quality), resulting in a biased presentation of those works to the public.
If Ao3 was truly an archive it wouldn't allow users to remove their works, and neither would it have features that result in biases of individual user's taste and preferences to guide the interactions with those work.
Ao3 could choose to actually be an archive and only allow the "orphaning" of fics (where the username is taken off the fic but it remains on the site) and, at most, private kudos, but they don't. Because they don't actually care about archiving material and presenting that material without bias.
Also, archives CAN and often DO have regulations and specifications on which kind of material they will and won't archive, including ethic based regulations and specifications. So "being an archive" is not an excuse to allow creative works that exploit real children, eroticize children, rape, and abuse in general as whole, or an excuse to welcome out and proud predators who get off to the idea of raping other people and or creatures to share their abuse fantasies on their site.
Ao3 isn't actually an archive because nothing is actually persevered, just hosted. It's a fanfic-centric social website dependant on user-driven curation via kudos, boomarks, and comments rather than an algorithm to divert attention. It's a social site where you can share your fanfics and get feedback on those fics via public bookmarks, kudos, and comments, and directly socialize with other people by commenting on those fics or replying to already existing comments.
Sure, the filtering system is incredible when people actually tag their shit properly, the lack of ads even better, and it may falsely call itself an archive while archiving nothing in actuality, but that doesn't make it perfect and free from criticism.
Also, just to be clear, while I'm anti Ao3 I'm not inherently anti the users of Ao3 as long as they're also critical of the harmful things Ao3 allows and facilitate and they don't donate to the website or encourage others to donate to it.
I understand alternative fanfic websites are harder to use due to invasive ads, censorship of bisexual and homosexual users and fanfics, and or just lack the same volume of users, making Ao3 preferable in those regards, and I sympathize with that.
By all means, take up all the room you want on Ao3's servers without giving them a single cent in return. I love that for you. Hell, I do the same, even. But the website itself is still deplorable though, and I will always hate it, its defenders, and especially its financial supporters.