Buffs are good, and follow the general path of how the game's going. He needs a total of 3300g to ramp into 100% crit. During this point, his damage is both extremely unreliable and low in a 1v1 situation. prior to this, he required either 3 items, or rune allocation to acquire crit, forcing his build into an extremely linear path.
He has probably one of the lowest base health pools out of viable midlaners. He gets absolutely booty blasted by bruisers and melee lane bullies. With the mana regen changes, most ranged mages can outpreform yasuo in a 1v1 situation, and hold that lead (AA to remove shield, spell reel when it drops).
The issue with yasuo isn't in how powerlevel, that's honestly never been an issue with him, the issue is in celerity and champ complexity. I've played yasuo a few times in the past month or so, I'm a garbage player, i'm really, really bad. Whenever I'm ahead with him, I notice my enemies following patterns that don't adapt to yasuo's play. When I'm loosing, enemies are adapting to his play and the flow of his power.
In this case, if I get freefarm and go against a passive laner, I will get my 3300g, cap crit, ruin your day, back for pickaxe, then gg, midlane is won. a good opponent knows that I'm extremely squishy under my shield, and windwall is on a 20 second cooldown. most mages can get in about 3 spell reels in 20 seconds with no cdr at lower levels.
How to deal with yasuo:
1: LEARN HOW TO PLAY HIM
this is the biggest aspect of countering any champion. learn basic combos, fakeouts, the powerspikes, the weak points. Yasuo plays the rice game better than nasus or veigar. You cannot outrice yasuo in the earlygame. Yasuo cannot safely trade with anyone except squishy targets before he starts building AD.
2: counterpick him
his laning phase is both incredibly crucial to him, but it's also his weakest point. abuse this. Lock him out of farm by playing a lane bully. Riven, darius, garen, renekton, fiora, and many others absolutely tear yasuo to pieces. Check out the fnatic vs c9 game where huni took yas against darus.
3: play carefully, do not overextend
So you fucked him up earlygame, don't get cocky, freeze lane, don't let him farm, don't let their jungler kill you. If you can, get your jungle in on the free kills.
4: do not let him roam
yasuo scales on gold, not ap, not ad, not crit, gold. Don't let him get gold and you win. If you see him roam bot, follow him, and stuff his roam up his ass and ship him back to base with a respawn timer, he has extremely weak base stats, and until he hits 3300 gold, he cannot build any ad or he's at a massive disadvantage. all his abilities damage caps out at around 250ish until post 3300g.
5: build against him
Randuins. -10% crit damage. Armor. All of yasuo's big damage abilities are physical, and everything but his ult, which is primarily just additional cc, is a crit. you should have at least 1 defensive item in your build, regardless of who you play, you see an enemy yasuo, you build randuins.