Mordekaiser was one of my favorite anti-assassins in season 3. His rework might let him do some cool new things, but he's stuck in a similar situation to Kassadin now: He will likely keep getting power removed from his kit over and over because of ONE broken mechanic
Seriously, just like how Kassadin's only power was his ult, the only reason Mordekaiser is still so "strong" is because of his goddamn dragon ghost which makes no real sense to have and is silly but just STUPIDLY BROKEN
Otherwise: His W prevents him from going into a solo lane and the recent nerfs have severely weakened his laning power (His cheesy W is the only thing that really let him win lane), his reworked E severely weakens his early farming, and only his Q really still has much power. He currently relies almost entirely on snowball because he can easily get kited and killed when the enemy is just slightly ahead of him.
Oh yes, he can come back due to his late-game scaling, but he's not his old self, he's not Vayne, he'll never solo-carry without MASSIVE mistakes by the enemy