He explodes. His damage is all tied into short range meaning he has to be in danger to deal it. Zed is far more toxic then Ekko hell most mids will devour him and if he goes top he will lose hard to a decent bruiser.
900 damage on his passive? and it requires 3 Attacks to proc. So it ain't that easy to get the damage off esp as a melee champion.
Oh and just so you know I am on the pbe and have actually played him.
I am not saying he isn't strong but he does have a lot of counter play and risk vs reward.
He is actually fairly balanced. People just don't know how to play against him. Keep in mind he is a ap Assassin. He can blow people up but not if you are careful with positioning and use your abilities wisely.