Yasuo doesn't seem to be intended to be an assassin as far as Rito I'd concerned; he's been lumped in with Fiora as a skirmisher/fighter rather than a hard carry type assassin. Problem with Yasuo is how his passive works, primarily in how it doubles crit.
Doubling crit means he's very gold efficient and should spike in power rather quickly. Higher crit also makes him much more reliable since he will be able to deal a lot of damage off of only two crit items; as long as Yasuo gets his items, it doesn't matter if he goes 0/15, he will still be able to carry fights without having needed to do well earlier. Moreover, he would reach his spike faster than any other champion since he makes items much more gold efficient via his passive. This was compounded by the fact that he could handle tanks with the use of his ult's % bonus pen. It essentially formed a situation where as long as Yasuo could stick to you, he'd kill you regardless of who you were. Hence the arrival of a sticky tank build with Mallet and PD. PD reduced damage from your target and Yasuo is auto focused and Q applies on-hit, so Mallet ensured you can't escape. Tri force helped with primarily the Phage passive speed up and the Sheen procs which made him deal much more damage just by auto-ing between Q's, with the benefit of both applying Mallet. Since he gets double crit, PD went from 30% crit to 60% crit instantly, meaning 3 out of 5 autos would crit vs 1.5 autos out of 5 criting for normal characters. Since Tri and Mallet both gave decent AD, the crits became meaningful (doubling damage on a larger amount is more significant obviously) in addition to never being able to run because of Mallet plus the high movespeed of Yasuo due to Rage + PD MS + Tri MS. Throw in two filler tank items and you had a champion who could almost certainly win any 1v1, split push fantastically, kill tanks, and dive backlines with relative ease.
Now obviously nerfs have happened to directly handle this particular build, but Yasuo will continue to be a problem as long as his passive remains that way imo. I don't think mechanics that alter gold efficiency are bad, I just think that they require a massive amount of effort to balance because they alter how items and gold efficiency function on a champion.
I think if they wanted to fully balance Yasuo without removing his double crit, they'd need to do something to reduce gold efficiency gained from his passive, like reduce the damage on crits more (maybe try 5% increments for testing), reduce the double to maybe 1.5 or 1.25, forcing Yasuo to build more crit items and pushing him towards a squishier build, which his ult change seems to indicate is the direction Rito wants to go. You could also investigate the interaction between his Q CD and building AS if needed, reduce confusion on build paths maybe. I'm not a balance genius by any means, but that's what I believe is the core problem. Not windwall, not his mobility, nothing beyond his passive imo.
