I think the only issue with Zed is that mid range waveclear puts him over the top.
Having manaless waveclear? Okay that's not an issue at all. Having 900 range waveclear that goes through all enemy units? That's where the problem lies. Compare this to Annie's ~600 range spells that cost nearly 100 mana, or Lux, being considered an extremely long range mage, with 1100 range on her E, only 200 range more than a manaless AD assassin. When you add to this the fact that this is on a 6 second cooldown and he has a very strong all in capability and insane mobility, this becomes a problem.
He has no clearly defined weaknesses right now. Looking at his shurikens would be a really good start. These are hard to dodge early game when he does it 10 times a minute and they can't be blocked by minions. Look at every other champion with strong waveclear/poke. If you jump on them they can't escape. Ziggs is as close as it gets to having poke and an escape, but his escape is weak and on a massive cooldown, and mana problems prevent him from going unpunished spamming his Q every second it's off cooldown.
If you jump a Zed, they just dash away. There's not a lot of weaknesses to exploit against him.
This is the problem with Zed. This item singlehandedly gives Zed a free lane against basically any ap, especially those who revolve around burst. Look at Syndra, the most reknown burst mage in the game aside from Annie. She has the lowest winrate in all of mid lane_. Simply because Zed can 100-0 her and Syndra cant because of hexdrinker. Even if Syndra has seekers, it doesnt matter. Her and basically all other mages just lose because of the immediate superiority of hexdrinker.