Voli has among the worst base stats of all "tanks", the only thing he has going for him is his passive. He doesn't even provide the hard utility that proper tanks provide, he can flip a single person but he can't knockup or snare your hole team. Kiting wrecks his day. Also, he's not really played that often and pretty much never in competitive, this has it's reasons.
Just because a champ is above 50% winrate doesn't mean they are OP, Voli has a fairly small and dedicated playerbase.
Also, his lategame scaling is more or less nonexistent, since he has no defensive steroids,. The only really good scaling he has is that of his bite and this is entirely cohingent on him not falling behind and affording to buy those crucial hp stacking items, any voli that falls behind is dead wheight since unlike true tanks, he melts so easely under focus fire, passive or no.
While his secondary might be tank, he's actually more of a fighter, like Jax, Mundo etc.