Recently, there's been a huge outcry from the community that champions like Zed are a problem because they have really high win+pick rates, to which most high elos say that Zed isn't a problem.
And then people scratch their heads and say, "remove the word elo, are you just high?" and there's a good reason for it. In most cases, it's because encounters in low elo are usually lethal, and Zed is really, REALLY good at being lethal. If you're a squishy there's simply no dueling him.
In high elo though (in theory, most games are too clown fiesta unless you're in a region like KR), one of the things that gets measured is kit strength. To give an idea of what I mean, let's compare Kalista to Zed.
Kalista has a lot of kit strength. She has the ability to enable an initiate, secure objectives, and she is a dominant laner. If I'm playing Ezreal in low elo, I might try to hit her with my Q's because if I weave in my spells perfectly, I'll outdps her. If I'm playing Ezreal in high elo, I realize that that is so unlikely that I tend to avoid interactions altogether and let the wave push to my side. Kalista's passive gives her a lot of strength in determining the early laning phase as well as lane priority for the jungler to play off of.
Another overly nerfed champion is Viktor. Viktor's E is what I call the CD tax. Most Koreans agree that in lane, you should never miss your laser, and so what happens is the champion consistently hits you with E's and you need to hope that he runs out of mana before you run out of hp. Then, he scales up and he wins teamfights extremely hard. This is another strong kit, and if you master the fundamentals of the game he's extremely safe because you know how to position the wave to never allow for a significant opening. To compensate, Riot has had to nerf the kit down to the point that only people with extremely strong basic gameplay fundamentals can play him consistently.
On the other hand, Zed has very little kit strength. As with all assassins, he's very scary in lane because if you don't control the waves right he can chunk you down for significant amounts of damage. That being said, early on he doesn't give good priority or lane presence, and his inability to control waves means he's playing around the capabilities of his opponent. He also cannot set up for a gank the way other assassins (namely LeBlanc) can. He is a strong duelist later in the game but he doesn't take TP so his map presence is weak and if you have a dedicated support it's hard for him to all in as well.
So what Riot has been doing is buffing his numbers. This only works for low elo and a select few matchups that he only barely lost, so it doesn't make up for the glaring flaws in his kit.
Ultimately, Zed is likely overtuned, and might need a numbers nerf to compensate for his frustration in low elo, but the glaring issue is more that people aren't paying respect to his kit. If you know how to manage waves to avoid all ins, the champion becomes significantly weaker.