Yeah there is just a lot that makes Agartha pretty bad, but I do recommend at least reading it because it will honestly help crystallize why people dislike it so much. It's one thing to see people complain about it and it's another to see "oh that's why people dislike it so much." If I were to have to level complaints at Agartha specifically, these would be my big ones:
- The one of Agartha is wildly different from the rest of the game. Agartha is, for a lack of a better word, very horny. This isn't inherently a bad thing, but F/GO is a lot less horny than Agartha is. While it was toned down the NA release (there are scenes in the JP release where Penthesilea talks about kidnapping you and using you as a stud), its still present. Pretty early on, there's a scene where Dahut has sex with a man and then kills him. It's very weird for F/GO to just, have snuff in it.
- Characters are wildly mischaracterized, both in the normal way and in the specific way the plot uses. Mash and Guda behave a lot differently in Agartha, and while there are some similarities, there's an entire scene where Guda and Mash borderline sexually harass Chevalier d'Eon for the sake of a transmisogynistic joke. Characters like Drake, Fergus Lily, and Heracles are altered as a plot point in the story, but they're done so in ways that either completely destroys their previous ties to the character or in the case of Fergus Lily, is the set-up for a lot of really uncomfortable humor and a tragically badly written monologue.
- The base setting for the world, the world of Agartha, is very, very sexist. It's a world where not only are the roles flips, where women are the oppressors and men are the oppressed, but it's done in a really comically exaggerated way, where men are routinely tortured or killed, or held in sexual/non-sexual slavery. It's not only something that the writing fails to interrogate at all, but it also specifically places the blame for this setting on Scheherazade's trauma. The world doesn't just exist, it was created by Scheherazade as a world where men could no longer hurt her. It's fucked up actually.
- Scheherazade herself is probably one of the worst written villains in the entire franchise, to the point where her plan makes remarkably little sense and kind of ignores a lot of the real world plot building of Nasu lore. It sounds clever enough until you remember there are two entire organizations who are extremely powerful and extremely adept at covering up evidence of magic. People are sending wasps on planes in this universe that turn you into zombies, Scheherazade's plan would not work.
There's only really one thing I give Agartha at all, and that is that its portrayal of Columbus is pretty good. He's a manipulative assholes who fakes having amnesia in order to get you onto his plan, which is to essentially have a never ending supply of asexually reproducing women slaves (?????????? as a side note. what.) He's a complete scumbag who regularly lies and withholds information even from the people he's trying to "save". It's great. He's the only good part.
5. WHICH LEADS INTO POINT NUMBER 5, WHICH IS THIS FUCKING STORY CHAPTER WAS WRITTEN AS IF YOU'VE READ SHINJUKU, WHICH IS REALLY ANNOYING ACTUALLY. This is petty but part of the gimmick of 1.5 is that you can play any of the 1.5 chapters in any order, and that works for three of them, but Agartha in particular is deliberately subverting something that happens in Shinjuku and holy christ I cannot fucking imagine how one reads Shinjuku AFTER Agartha without knowing the plot of either. Honestly I imagine it has to ruin the fucking experience. Fuck off Minase.
Overall, its a mess. Even if you want to stick it to people like me who don't like the themes of the Lostbelt, it's really poorly written for the most part, and what good moments even exist after often overshadowed by extremely bad writing (there's even a Deus Ex Machina moment at the very end just as a cherry on top).