By definition Braum is a warden, defensive tank meant to lock down any incoming threats.
In short he stays back and protects the ADC and the mage in teamfights with guess what, his CC and his shield.
I can tell you my frustration playing Warwick and not killing the opponent ADC once because Braum hardcounters divers and any engagers in general.
Now, since I told you what Braum is supposed to do, let me tell you his weaknesses and why he isn't broken:
1)Immobile
2)bad vs AOE
3)Bad vs any sort of consistent poke as he can't heal the ADC in lane or teamfights