Error Margin and why Zed is the most 'healthy' assassin
Disclaimer: I don't particularly like Zed, I find him extremely annoying - however I find complaints about him being less toxic than alternative assassins to be faulty and ill informed. The last thing I want is for Riot to take these nonsense ramblings floating around the community about 'high risk high reward' assassins seriously
Assassins are an extremely difficult class to balance, those of you who don't remember Akali/Kat/Talon in their primes should consider yourselves lucky. The entire premise of an Assassin is a champion who's basically worthless other than for killing other Champions reliably, which implies that the Assassin's team is at a disadvantage until they can kill someone.
This is fairly evident in 'normal' assassin:
- They have essentially 0 CC/utility
- They are extremely squishy
- They have inferior waveclear and poke (yes, even Zed)
- They can't do anything without going all-in
- They have almost no pressure outside of their potential to kill you
As you can imagine the qualities you want in a 'pure' Assassin are reliable assassination (AKA point and click with absurd upfront damage) so that they can easily capitalize on opportunity, as well as and a huge amount of base stats and snowball potential to offset the possibility of being useless. Because of the black and white nature of of "you either clicked on them and killed them or you didn't", these kits are balanced on a hair's edge around numbers which leaves very little margin of error - you either have the stats to be a monster or you're worthless.
**In comparison Mages are probably one of the easiest classes to make 'healthy' because they probably have the hands down largest margin of error. ** Being largely skill-shot based is a double edged sword that rewards good players and punishes nooblords. Unlike Tanks/ Assassins/ Bruisers who are balanced around a straight forward ball of stats that Riot deems as 'good enough', Mages are balanced around Riot's approximation of what % of spells a mage is expected to land. As a result, if you're ahead on a mage but suck at aiming, you're just as useless as a feeder. On the converse, if you're a huge feeder you can make up for your failure by sucking less and landing more spells as the game goes on.
- Mages have huge marginal value. Their job isn't limited to rigid guidelines like 'Be tanky enough to frontline' or 'kill people instantly', but rather 'do as much damage as possible' and 'cc as many of them as you can'
- Even if ahead, a mage can be outplayed (e.g. dodge shyt)
- Even if behind, a mage can outplay (e.g. aim better)
So what the fk does this have to do with Zed? What I've been getting at is 'margin of error' is a huge part of what makes a kit healthy. It allows players on both sides opportunities to counterplay and make comebacks, whereas static kits like Talon are for the most part extremely straight forward and balanced on eggshells.
However, because he's an Assassin and not just another mage, he needs other sources of forgiveness than 'land more spells'. What comes is as a result are Riot's 'LC$ Mechonix' balanced Champs:

Instead of revolving around pure numbers, these champions have unique mechanics (namely high mobility) that give them power in forms outside of stats, but ironically players seem to hate these champions for the very reason why they're probably less toxic than their alternatives. It's really pretty hard to add too much counterplay into an Assassin without making them useless (and Zed already has an exceptional amount of counterplay) so the next step is just to make them mobile and forgiving
- Mobility allows them to extend further and play riskier, which compensates for their relatively low stats
- Delayed damage / skill-shots / wind-wall/ whatever weird mechanic gives more counterplay options to both sides
- Safety/ utility/ poke/ range/ whatever give these champions marginal value rather than simply 'OP vs useless'
TL;DR: I don't think all this Zed QQ is merited. You could ask him to have less poke or be less mobile, but what you'd probably end up with is 'Zed presses R to deal half your health'
I think these mobility champions have enough flexibility and counterplay in their kits that they can afford to be slightly overtuned or undertuned and not be gamebreaking. They're really not broken and I really don't want Riot 'fixing' them.