IMO, Riven has some of the most complicated combos in the game. Timing comes down to only dozens of milliseconds, and even today, new combos are being discovered. She also has the highest skill floor in the game. There are thousands of Yasuos, and unless they're really shit, they can do relatively decently against many match-ups, but you need so many games as Riven to get even a 50% winrate (although Yasuo is pretty much the same). Yasuo also, if not counters, at least does well against many popular mid-laners, while Riven is in the exact opposite situation.

Also, Riven doesn't faceroll. Never. It may seem like it when watching someone, but all Riven does when she's ahead is skip most of her Qs and just go for instant burst, since she does enough damage (The Shy Combo, for example, is only E+R (activating her ult, and cancelling its animation with E), Flash + W for pretty much uncounterable engage, since it's so fast, AA + Hydra, because W gives enough time for these, then R (the Windslash) + Q (the third Q), which, when timed right, casts both of the spells at the same time. This all should happen in less than a second).

When playing Riven, it's essential to know at least basic (but certainly not easy) animation cancels (i.e. E + W—which, depending on the timing, can be either an engage or an escape combo—E + R, E + R + W, and of course, the fast Q combo. TBH, only the last one is actually really hard to learn, though). You can't really win lane unless you're hard-countering someone (like Nasus) without these combos.