Morde is Less Reliable
So I'm a Mastery 7 Mordekaiser.
The reason I played so much of old Mordekaiser was that he was a strong counter to AD midlaners like Yasuo, Zed, Talon and them. Actually he was a strong counter to any melee ranged midlaner. Katarina, Ekko and all of them included. He could even fight against some melee toplane champs like Riven, Jax and Renekton.
This is because while he couldn't really stick to the enemy w/o the help of items once he was in the thick of it, he'd be able to do his damage.
His old Q was a 3 hit passive change to his auto attacks. His old E had very low range but came out almost immediately. His old W came out immediately and would surround him and the minion/champ he chose.
He had counters in that if your champ cleared the wave well he wouldn't be able to use the DOT aspect of his W which was essential to his kit, and that the third hit of his Q was the hit that mattered the most but he had low movespeed and had a hard time sticking to enemies.
New Morde is not as reliable.
New Morde's autoattacks are the only reliable damage in his kit. His Q comes out quite slow and if you're fighting a Sylas, Renekton, Katarina, Graves, Akali or Ahri or anyone with a dash, it most likely won't land. Anyone with a movement speed steroid like Teemo or Blitz will just be able to move out of his E.
Honestly, I don't see him being bad because of his teamfight presence, and if you time with the enemy farming you can land your abilities, but I feel like new Morde doesn't have a lot of what I liked about old Morde. The healing aspect is a lot less so you can't lure the enemy into a false sense of security in a fight. I can't heal my jungler clearing camps so they stick around for a gank, and just... a lot's different about the champ.
I kind of wish my mastery points were cleared because this does not feel like the same champion to me, and while I'll still try to make him work, I think I'll stop playing him as my reliable pocket mid.