Ben Affleck is a good director. Very good even. The Town and Gone Baby Gone are both really good movies.
I haven't seen any evidence that he can do really big budget action sequences though. But that doesn't mean he can't, after all there was a time where Joss Whedon, Favreau, etc didn't have that experience either.
Justice League needs that larger than life bombastic quality. It seems like such a massive change in style from what he's done previously though.