Remember when Irelia was OP? and morello said she was a problem SPECIFICALLY because of her kit? And Xin was nerfed over and over, he's in an AWFUL place atm, heimer? never heard anyone qq bout his turrets, maybe his rockets, but not his turrets. Talon could litterally spam his rake without running out of mana the entire lane phase. along with it hitting for ridiculous amounts.
Voli need's to be looked at in some way or another. His passive is mundos ult with half the amount and twice the cd but no real cost, his Q is singed' fling with a shorter cd, his W is basically garens ult, and his E is one of the strongest slows in game along with being pretty lane phase changing with the minion fear.
And its not just strict mathematical issues like that. If you have abilities that synergize really well and help you accomplish something all together, they can be more valuable for you than they would be individually if your other abilities were different.
For an easy example, imagine if we gave Blade Surge to Tristana. She wouldn't get nearly the same value out of it that Irelia gets.
Volibear's kit is very tightly integrated. Everything works very well together, and most of it scales off of the same things, so its easy to itemize him and you don't miss very much in the way of ability scores you'd like to buy, but can't fit into your six item slots. This makes him more powerful than his abilities might seem to make him if you consider them one by one.
His balance can still be adjusted by adjusting his numbers, obviously. The OP is right about that. But kit is part of the equation.