Yasuo is basically designed to be a melee marksman/ADC. Funnily enough, marksmen don't do too well in melee range because of their defensive stats.
Like ADCs: Yasuo has low base HP, relatively low HP regen, no scaling MR, and even lesser offensive stats.
Compare him to Tryndamere, another melee champion who shares the same roles as Yasuo (Fighter/Slayer). At level 1, Tryndamere has:
* over 100 more HP than Yasuo, 626 compared to Yasuo's 518
* two more HP regen
* 6 more AD
* equal attackspeed
* 0.6 less Armor
* 2 more MR (that will then scale with level where Yasuo's will not)
Now compare him to the upcoming ADC, Xayah, who has:
* 27 HP more than Yasuo, 545 compared to Yasuo's 518 (she's not even the only marksman with more HP than Yasuo)
* 0.51 less HP regen
* 0.6 more AD
* 0.05 less attackspeed
* 0.7 less armor
* equal MR (no level scaling, of course. Just like Yasuo)
Notice how Yasuo's stats are roughly equivalent to the new marksman but considerably lower than Tryndamere's, a champion who one would think is pretty similar. Yasuo's health doesn't even catch up in the late game, where he has a low 1912 at 18 (almost 400 less than Tryndamere).
So now let's look at his shield. It blocks 100 damage at level 1. Without putting an ounce of thought into it, one might say that this shores up Yasuo's HP discrepancy with Tyndamere and other melee champions, and they'd be half right. You have to take into account however than even a single point of champion/monster damage will pop the shield, making it fade away completely 1 second later. If we consider his shield effective HP, any champion with a ranged auto attack can effectively hit him for 100 damage regardless of their actual damage, then unload on him a second later to hit his actual HP directly.
Yasuo has this shield because of his low HP and HP regen, and has clear relatively easy counterplay.
What's the deal with his double crit chance then? Whiners often ignore the fact that he takes a penalty to crit damage for it, a penalty that applies *after* Infinity Edge's bonus making it a less effective item on him than every other champion in the game, but even ignoring that there's good reason for it. Yasuo, as established, is as squishy as a marksman. Generally, marksmen don't like being in people's faces because they often enough die before they can be useful (that is, deal a significant amount of damage). Marksmen mitigate their weakness by, wait for it, *being ranged*. By being at range, Marksmen keep out of the range of high damage melee champs and have more time to react to skillshots and other big damage from things like mages or even other marksmen. Yasuo does not have this advantage however.
Generally, Melee champs have higher resistances and often also build more resistances so they can stay alive longer in the thick of things and put out as much damage as possible. Seeing as Yasuo does not have these increased resistances, it only makes sense to instead make him do more damage accomplished with his double crit. Yasuo does not have the luxury of plucking away at your HP from range while he waits to finish all of his items like marksmen do, so instead his build "turns on" more or less after two items compared to three or so like marksmen.
If Yasuo did not have his double crit, he would not do enough damage to be worth picking or they'd have to give him some other source of damage. A bonus effect of this double crit passive being his main and really only source of big damage, is he's pretty much forced to build crit (especially when you take his ult into account). Because of this passive, at the very least two of his item slots are locked into being offensive items, and to keep relevant damage with his lowered crit damage he has to keep building offensive after that. Remember bruiser Yasuo? How about tank Fiora? I don't think anyone wants him to do either of those things.
As a bonus, here's a quick blurb on why he has the much complained about Windwall:
If he didn't have it or something like it he would die moments after diving into an enemy team or even before he can reach enemy mages or marksmen or just about anything that does high damage. Because his dash is unit targeted, if his target is out of range and he doesn't have a string of minions to close the gap, Windwall is his only hope of surviving an approach on high damage enemy champs particularly in the late game where he'll have few or no defensive items to mitigate damage.
So there you have it. That's my analysis on why Yasuo has all those "free" stats people complain about. Hopefully it opens some eyes but I'm sure most people would rather keep their eyes shut and blindly rage about Yasuo instead of learning and understanding.
If you'd like to shit on me some in the comments, I'm a Yasuo main in norms who's never made it past low Silver in ranked. I mean, that's pretty much average elo but I'm sure my opinions on balance will be disregarded for it anyway.
If you'd actually like to learn and get some tips for playing against Yasuo, I'm happy to give some tips.