When juggernauts were introduced to the game, the whole concept was that they would be comparably immobile, somewhat tanky, kind of hard to disengage from, but could deal large amounts of damage to the enemies if left alone for too long. They would have a large health pool, pretty high defenses, and the highest base attack damage in the game.
Back then it worked. Frankly it still does.
But Mordekaiser was somewhat overpowered, bringing about a new "meta" with him being the "adc" on bot lane, dominating the lane due to simply out-sustaining even ranged champions that he couldn't actually reach to poke, then proceeding to take drake which was hard to contest against since he was so strong back then and generally start snowballing until the "snowball" was large enough to consume Europe. His core build essentially being Sterak's and Trinity, he could dish out huge amounts of damage during the mid-game while it was pretty tough to get rid of him, and his W steroid was making things even worse.
So naturally he was nerfed, fairly.
Now we go to patch 8.12 where Sterak's interaction with Sheen has finally been removed. This means that any champion who used this combination of items has been nerfed. Clearly some of those champions needed the nerf (looking at you Irelia) but others... Not so much!
Mordekaiser's current state places him in a weird position where dealing damage to the enemy (if he manages to get in range) has absolutely no benefit in lane, since the damage he shields from he basically takes anyway due to the way his kit works. So poking is out of the question. He has no real sustain anymore compared to other "in meta" champions. He is fairly squishy compared to many. His only way to reach enemies to deal the three autos is clumsy to say the least. Building AP on him is no longer really cost efficient since his scaling kind of sucks. And if you build tank you are pretty much useless.
I can't think of any match up he would do well against at the moment. In any lane.
I think he needs to be brought back to life. Get it? Cause he was revived according to the lore? It was a pun... Never mind! :P