So played mid a few times against an Ekko obviously. My biggest issue with him is that when he reaches 6 you can't kill him in lane. Things go bad for him and he just ults to safety and you can't stun/disable him.
Played Zed and obviously, my early was better and i didn't have much trouble with his mobility and damage, unlike the ashe at bot that got oneshot from time to time. Didn't have trouble with him mid and late either. I just can't stand an ult that gives so much safety and has damage as well. It's cooldown isn't exactly long either. Ekko can't really ever lose his lane. You might stop him from becoming fed but you will never gain an advantage over him.
I kinda see him having the same problems Lee had. Hes too good at too many things. CC is very good, his mobility is good and even better when he manages to land his AA's and is pretty hard to kill even without his ult. But unlike Lee this guy actually scales pretty well into late.
He also seems to be a pretty good jungler with great ganks. But he seems to do very bad on top against the aggresive top laners like Renek, Maokai or Riven.