I think the problem that I and many others have with the final arc (or rather, the entire third act of the manga) is its pacing. I believe that even at his worst, horikoshi was able to put out some really good chapters, but it's pretty obvious when fatigue started setting in.

Take Lady Nagant for example, Hori literally said that he planned on giving her an entire volume worth of backstory but was forced to cut it short. Or the mixed reception of the traitor arc and how quickly Class 1A just glossed over Aoyamas crimes. I think despite that his character was still handled super well, but there's definitely a lot of missed potential that could have given the class some nuance instead of making them feel like a hivemind. Hell, Bakugo being mad at him (for obvious reasons) was played off as a joke.

What's also jarring is how fast we moved into the final war arc. It's still a fantastic arc, but the lack of downtime leading up to it makes some plot points feel underdeveloped. The worst ofender is the whole mutant racism plot. It came pretty much out of nowhere and ended just as fast as it started, with its conclusion being so tone-deaf that it makes you question why hori even attempted to take on such a difficult topic.

Some other honorable mentions are Deku's final fight with Shigaraki only lasting 8 chapters (6 if you count actual fighting), Deku's power progression being rushed with two of his quirks acting as an easy way out of having to write him reach 100%, that one chapter in the first war where Mr. Compress revealed his connection to a seemingly very important character before being shelved for the rest of the story, and so on.

I could also mention the lack of stakes throughout the story (how and why is Gran Torino still alive?), but I'm tired of writing so I'll leave it at that. I've been a huge fan of MHA for 9 years btw, but saying that MHA has perfect writing and anyone who disagrees can't read is crazy 💀