Zed is actually fairly balanced now.
He has the slowest assassination combo to trade for his safety. Other assassins such as Talon can literally kill Zed before Zed pulls out a full combo on Talon. Heck, even CC mages such as Annie, Cassiopeia, or Ahri can annihilate Zed now due to the 1 second ultimate return delay, giving them a point blank opportunity to stun.
Rather than being a go to pick, people actually play Zed now for his strengths. He has spammable poke and an oppressive ultimate early, but later on, he gets outclassed or countered by many champions.
Yasuo on the other hand is in fact tremendously under-powered by himself.
Many mid lane champions won't even give Yasuo a chance to charge up 3 Q's late game. A full combo from Viktor, Annie, Fizz, etc. and he is done.
Yasuo is essentially an ADC at melee range with ~30% more damage.
However, pair him with Malphite, Alistar, or Zac and you get an obscurely broken team fighting machine. Yasuo is tremendously weak against most mages, assassins, and bruisers. However, given the chance, he can and will hit them with an AOE nuke and kill them before they can retaliate.
As a general tip with fighting Yasuo, I see many players make the mistake of standing in their minion wave. The tutorial recommends it to avoid skill shots, but it is detrimental to do so against Yasuo. Albeit most Mages can kill Yauso in one combo, standing close to minions give him a chance to kill you as well. If you by any chance miss your abilities, it is a free kill for him.