Personally, I feel as if there’s a lot Horikoshi wants to do but just doesn’t, whether that would be due to a lack of knowledge, experience or just time. He may very well have bitten off more than he can chew with HeroAca.
For example, characters: HeroAca is primarily set in a school. So that’ll have a lot of characters right off the bat, but then you have Pro Heroes, villain factions, civilians, sidekicks. And even within the school, you have two whole Hero classes, a support course, a general studies course, and The Big Three. It doesn’t help that Horikoshi’s actually really good with character designs, so naturally people would want to learn more about them. But Horikoshi doesn’t have the luxury of putting a lot of school antics in order to explore the school well cause this is a battle shonen, and people are here for high-stakes fights - the school arcs tend to be the more divisive ones in the fandom. Deku’s meant to become the greatest hero as well, so we have to focus on him the most. Horikoshi’s also drawing parallels between him and Shigaraki, as he’s on a journey to become the greatest villain and as such is a sort of protagonist in his own right. So he obviously needs some focus, along with his gang as well. All of this, and there’s only so much you can draw in one week (Horikoshi does less pages per chapter than usual, too) and you can only linger on an arc for so long. All of this has the unfortunate effect of MHA not really managing to develop many of the characters that it introduces, and the even more unfortunate effect of its female characters generally just barely getting any spotlight. It’s something I do hope the series finds a way to fix, somehow.
Other currently running shonen (Black Clover, The Promised Neverland, Chainsaw Man, Kimetsu no Yaiba) have a more focused cast of characters, which makes them easier to develop more. This might honestly be the preferred method for writing characters. Not every manga is One Piece.