He's not the only champion with a lot of mains yet he's the only champion that literally goes even or beats EVERY SINGLE JUNGLER in the game at nearly all stages. Even his late game is bonkers. Because of his mobility and stealth, he basically can't die unless he screws up really hard. He becomes a split-push god and also a menace in teamfights with his crazy damage, CC from boxes, disruption etc. He has the highest winrate of any JG late game at 56%+.
I am annoyed by a lot of champs that I think are borderline OP, such as Udyr. However, Shaco is the only one that feels truly unfair to play versus. He has no true weak point and I've even seen some build a bit tanky and be effective.