Out of every champion ive observed over the last year, I feel like Darius stands alone in this regard:
Mid to late game, if you focus him, he is tanky AF, and his heals up like crazy too. Not a good decision.
BUT if you DON'T focus him, he does high damage and can finish off several of you with his ult.
Given how he people build him, I think he does both way too well for it to be considered balanced.
There are many bullshit champions, and Darius of course can lose, but the fact that I've seen him in this spot so many times, with my team not knowing what to do about him, is a problem to me. Its too much, and its too much cheese with that repeatable ult.