Morde is Fundamentally Broken, and Riot will not be able to balance him with his current kit
In general, I think people who say a champion is "broken" are wrong, because I think a champion can maintain their kit and still be balanced. Tahm kench, for instance, is overtuned right now because he can outduel too many top laners, but there's nothing conceptually wrong with his kit, at least in my opinion. However, I think morde is different. This is because so long as he maintains his gameplay identity as a juggernaut with his current ultimate, he becomes simply too hard to beat, even conceptually. The point of a juggernaut is that they can beat you to death if you stand next to them, but have a harder time getting close to you - Riot could nerf morde's numbers to the point where he couldn't beat other melee's in straight 1v1s, but I think he would no longer be a juggernaut in that case. When you combine this dueling potential which is usually gated by being hard to capitalize upon with morde's ult, which literally forces a duel with an enemy champion (including a ranged one, who would usually be able to kite you)... what do you do?
running away from him and still doing no dmg, if you get caught by his e its grey screen heaven, to be honest no champ in a moba should be able to stop interactions with other champs and mord at this point is one of those champs. You cant duel him, you can barely do dmg to him and he makes farming a nightmare because if he procs his passive he can stand in a wave and use his passive to farm and he has no cds so if you go in to farm just know all his spells are off cd and there is a 98% chance he is going to kick the shit out of you if you dont pop the flash away. Did I say you better hope he used his before you try that move?