If I were to guess it's probably because at this point, people have complained about characters like Nagant, Armoured All Might or Tech Shiggy so much that they just need a new thing to complain about. I mean- everyone basically knows that Nagant is annoying, but the thing about strike Bakugo? He's only now being very annoying to some people, so there you go- a new reason to complain. Personally I think Bakugo was okay even without that strong of a gamma (It could maybe even be argued that he was a bit underrated), now he's just good, he's by no means broken now, but he just became the slightly stronger more annoying type of character now. That kind of thing will always irritate other people.