If i recall right, Yasuo was intended as melee-carry, so a primary damagedealer that kinda fills the role off an adc(and with his kit he actually is able to go botlane instead of an adc), and with rather low range mobillity(if you have no setup for the ult) and beeing melee, he needs to get some tankiness to do something in teamfights, so so beeing able to deal with tanks seems needed for him as he usually can'tafford the itemslot for a LW.
Maybe 50% bonus is to much and a total pen would be better as it wouldn't mean building armor vs yasuo feels totally usless...
Anyway, i personally think Yasuo would be a more interesting and health champ if the crit was removed from him and therefor the nerfs to his shield get undone, maybe with a small ratio buff to his Q. This would translate to a less frustraiting powerspike(
+
is quite overbearing on him), and he wouldn't fuction as an assasin by bursting down squishy befor they touch the gound with the Q+R+AA+Q combo anymore, relying more on sustained damage and his mobillity paired with his shield.
Another wierd thing is that he gets the pen after his ult, though he usually deals with tanks till he uses the ult, as usually the ult is his acces to the squishy backline.
this leads to the feel that his ult mostly prohibits getting armor on squishy to counter his burst.
If you face talon or Zed, you usually get an early seekers as mage, and a zhonya later on, and you feel good about it.
As adc, you often would want a GA, or even a randuins as it's the most efficient single-def item vs physical damge, and with an ad-midlaner, chances are that the enemie team is very ad-heavy.
Vs Yasuo, those defensive options feel bad as he gets the 50% pen whenever he finds an engage...