Well, I'm not even gonna complain about the big picture stuff in that it's very obvious the dude didn't really plan all this shit out and it seems that he's making it up on the fly because that's not his fault. Pretty much every manga in this genre suffers from that and that comes baked into the cake, and also it's not even that bad.
The dialogue and scenes though are just getting worse and worse with each season. Again, I've only watched the anime, but some scenes are just baffling. Off the top of my head, the scene where the team is rescuing Eren from the german bros is just so stupid and everyone acts like a complete retard not asking the obvious questions. Then you have the scene where christa or historia or whatever the fuck that little shit's name is is going to eat Eren, which is just an incomprehensible mess; like the dude just started writing that scene and let it run to see where the characters ended up with, but then at the end he didn't go back to rewrite it to smooth it out. It seems like a rushed first draft of a scene. Actually, come to think of it, literally everything to do with historia is astonishingly bad.
Even the dumb cheesy anime lines are fucking cringy because one the cool things about 1st season is that even though it's a shonen it didn't really have much of that shit>Large female titan eats mentor figure in front of eren and mikasa>Eren cries and can't transform>then he fully heals>turn to mikasa>I will wrap that scarf around you as many times as you want
what the fuck is that? I honestly just burst out laughing at that. And I'm not shitting on the series. What the series is good at is still great and keeps me going, but it was really disappointing how the later seasons turned out in that regard.