Assassins are also supposed to have counterplay. Zed's ult is one of the least counterplay giving skills in the game, yet it does a significant amount of damage. Zed ults you which is a targeted skill that you can no longer do anything to avoid. Then, he pops a shadown and unleashes both his damage skills on you with the boosted damage from his shadow, which takes pretty much the same amount of time as an autoattack. Then he reactivates ult and goes to safety meaning the amount of damage you deal to him is limited to one second and based on if you can accurately guess if he's going to stay behind you or use his W to reposition himself to throw you off. Hell, even without the bonus damage from death mark, that itself is a pretty powerful ultimate, but there is still the bonus damage from death mark to double what he did in that one second time frame.
Now look at all the other assassins. The next one with the least amount of counterplay is Rengar. But after Rengar ults in, he doesn't get a get out of trouble free card. If you're smart enough to stay with your team, they can punish him for it. In order to punish Zed, they have to know exactly who he's going to ult and be close enough by (he'll take any squishy, not just the adc). Then they have to know which side of the target, he will appear on, and that's not easy since the target doesn't always run in one direction in that panicked state. Then they have to be able to predict if he's going to use his W or not to reposition (which is more luck than skill). Then they actually have to hit their CC. If they miss any step along the way (which is more than likely), Zed just walks away free.
To balance him, they should make it to where he can't use R again to escape for 0.5 seconds after taking damage or till death mark's damage hits. That way your team can actually punish him by constantly doing any small amount of damage to keep him in place for a few seconds. He'll have to choose his targets based on positioning rather than on just whoever is squishy.
