The Volibear problem.
He does no damage besides his W, which is on a long ass cooldown unless you bite a minion or a monster. His ult doesn't really work with the rest of his kit, if anything I think it was just added because of lore.
Besides that, his Q and E feels super smooth and work well with eachother. He's more of a utility/gank tank jungler, but I just feel like he needs more mmph in power somewhere else. Yes, his W does a good chunk of damage, but he can only use it once per fight. After that, you kinda just walk back and forth until your Q or E is back up.