Hes not OP without grievious wounds. He's functioning as intended without grievous wounds. Hes more or less worthless with it though.
A single item or summoner spell completely negates Mundo's entire play style. He's situation at best, but still completely outclassed by any of the current top lane meta bruisers simply by being hard countered by the summoner spell that the enemy mid-laner is almost guaranteed to have. He gets destroyed in lane by any champions that can force trades with him. Force him to build armor first and hes in an even rougher spot. He has no escape and is pretty easy to gank. His weak lane, easy hard counter, and situation usefulness just make him a bad overall pick and it reflects in his win rate. Normally I would say win rate doesnt dictate how healthy a champion is, but when his win rate is constantly hovering below 45% on a champ that is super easy to use, it does say something
I am curious as to which other "other heavy sustain champions" are OP without grevious wounds though. Eleborate