Edit: Two points
A) I stopped playing Zed and went up 4 divisions of Plat
B) I just watched Zed fail to deal any damage to a 400 HP TF using his entire combo because TF had Seeker's Armguard
Clearly a great champion
He just doesn't cut it.
I've been trying to make him work at Plat for some time now, and I give up.
Zed is supposed to be a glass cannon assassin who can mid or top. Use your shadows to harass and make plays. Take out a key target in a teamfight and escape.
Except you cant.
No matter how fed you get, you cannot carry a game as Zed. sure you might be able to instagib a carry in a teamfight mid/late-mid game, and do some aoe damage, but you will quickly become a ninja punching bag.
His laning is absolutely awful until 4-6. You do 0 damage to anyone that opens Cloth 5 (which everyone can because hur dur Seekers Armguard), your W is on a massive CD.
Your Q is hilariously bad, being narrow, slow and short ranged. You rely on W/E combos to harass because of how unreliable your Q is.
You cant duel bruisers in lane, and your primary harass method is also your escape. And on a 23 second CD. You are forced max W second because A) your Q is terrible and B) The CD and energy return suck until you get levels in it. And you are reliant on it for harass/gapclosing/escaping.
In lane against an AP champ, they will simply start cloth 5, rush Seekers Armguard and Zhonya's, and you will never be able to trade with or duel them. All you do is waveclear and try to roam.
Then we move to teamfights
Most teamfights usually boil down to this
You ult the most fed carry
They get 300 points of shield/heals from someone
You EQ ignite AA and then get stunned
E again and try to W away, usually dying
Most of the time your target survives unless you are massively fed
Carries build warmogs now, and Zed needs AD to be relevant. He suffers from Talon syndrome of being poor in sustained combat (Dont even try to tell me his E is good for sustained combat, you're OOE after 1.5 combos). They will likely also have a Bulwark on their team.
Oh and lets not forget
[CENTER] Zed as a champion is completely countered by QSS, Zhonya's, and Intervention. [/CENTER]
If the enemy team has any of these things, you will be completely useless. The only thing you can do is splitpush. Until they send a bruiser with a decent gapcloser/sticking power and kill you anyway.
Zed has poor CC, his W is on a massive CD, he is ult reliant and is gated by energy more than any other energy champion.
Even with his increased popularity, his winrate is subpar as ever.
There is no reason to pick Zed in any game over Akali, Diana, Katarina, or most AP mids.