The problem with Riven
That people choose not to learn how to play against her.
For TLDR, read the first sentence of each paragraph. Before you comment a disagreement, read the paragraph you disagree with. If you have nothing to say on the matter, please refrain from posting (such as, trying to use rank as a way of discrediting).
She requires a pretty high level of skill and game knowledge to play. Yes, she has animation cancels that let her do about 30% more damage in less time when you are good with her and spend most of your time onetricking her. Sure, some people can pick up riven and do decent with her, however there is still a very clear difference between a person who does well with riven, and those that actually know her.
She also is fairly easy to counter by any champ that has displacement cc. Volibear for example has two ways of knocking her out of her third Q and his E also does more damage when she is in the air. She is also susceptible to ranged damage and poke. If she picked riven INTO you however, then you still SHOULD know to play a bit safer early level unless you are able to do consistant damage..... like Darius starting W, or Olaf starting W or Trundle Q.... etc. There will always be champions better early. There is many champions like this, and guess what? They are also being complained about..... why? For the same reason as Riven which is will be explained in paragraph 3...
Riven is outplaying you. It is not "stat check" most of the time, it is purely and simply they are outplaying you. If you flash to dodge the third Q and she flashes and still gets you? She outplayed you. If you literally were standing still and looking like you had your thumb in your mouth holding a blankey waiting to be tucked into bed? Guess what? The player is way above you in skill level. Therefore, that player would probably have beat you on nearly any other champion. Maybe not as hard, maybe harder.