Zed's combo is pretty healthy, especially for an assassin. His 3 second R detonation and general reliance on whittling foes down with poke gives a lot of room to deal with him, and having a lot of ways to counter someone's play is pretty much the definition of a healthy combo.
Issue with Zed is mostly he's been an omnipresent champion for ~3 years at this rate, being a frequent pick in every Elo and the professional scene.
There comes a point where every player would rather face champions with unhealthy combos and see some variety over fighting the same guy over and over and over and over again.
Also the huge early-game kill pressure boost this seasons Masteries provide generally give less time to react. The keystone masteries are crazy powerful, and there's no cheap defensive stats to pick up for anyone that isn't going full-tank, which cuts into Zed's early counterplay.
Zed certainly deserves a nerf, but not for his assassination pattern. There are plenty of worse assassins. Which is pretty much all of them.
is also literally the worst.
OHKO someone with essentially no warning. At least you can see LeBlanc and Fizz coming and clench.