This is one of the main things that always bothers me about balance debates. And it's reached a peak with Volibear.
It is physically impossible for a champion to be overpowered based on kit alone. It's all about the numbers.
This can be seen time and again. Look at Xin Zhao. On release, people were "Zomg! He can jump on your face and knock you up! Impossible to escape him! And if you try to fight him he nukes you and becomes tankier with his ultimate. Kit so broken, need to take away that jump." Or Heimer "Turrets so overpowered, he can farm lanes he isn't even in while fighting in others. He can push two lanes at once! Turrets need to be removed, or at least have a range on them!" back when he had three.
Saying Volibear's kit is overpowered is just stupid. Imagine if his slow was 20% at all ranks and his W did a third as much damage. Would anyone be complaining about him being overpowered? No(because he would be useless at that point), but his kit would be exactly the same. Just with some minor number tweaks.
Also take a look at base stats. How many times has an overpowered or underpowered champion been put to a pretty good place with just some base stat modifications? Talon was cried about so hard on release until they nerfed his mana regen on an AD assassin, and suddenly he doesn't have infinite mana anymore and he's good. Despite people, on his release, saying his kit made it impossible to kill him, too much mobility, etc etc.
Hell, I remember a year ago hearing people talk about Tryndamere. Always saying "Chicken such a bad skill, the slow is not going to hit anyone and the AD reduction doesn't matter because anyone standing to fight you is going to lose the DPS contest anyways." Now that he's popular again I see "Zomg! AD reduction soooo OP!!!" When the skill saw next to no change in that time. People seem to just like trying to use anything at their disposal to make points, contradicting themselves constantly.