Batman is a very, very good character. Let us not be mistaken, here.
However, he has weaknesses that other, higher tier characters do not. Specifically, he is only as good as the person playing him is at managing his trait. Trait management isn't as braindead as everyone thinks. Also, he actually does have to work to open good players up.
It's true he can keep people in long block stun with trait, MB Batarang, and his quick strings. However, he has no real overheads that can't be fuzzied, and a patient player can block him for some time.
He's good, very good, but not the best in the game. He's the Ryu of Injustice.