When somebody asks "can x beat y" there is, sometimes, an objective answer.
You assume equal skill, and you assume natural weaknesses, and you check numbers for dps. And you can see who will die at any point in the game assuming perfect play, and you will get an answer of who loses.
For example, Kog'Maw will lose 100% of the time against Master Yi in the late game. It doesn't matter what Kog tries, Yi will out damage and gap close him, and there isn't a thing he can do about it.
Now when you take a champion and compare them across all points of the game and you come up with lackluster results at nearly all points, you have to stop and question whether than champion is actually as good as you think.
And Yasuo underperforms hard in early game, slightly below mediocre by himself at mid game (increases based on team comp), and slightly above average at late game (increases based on team comp). You can't tell me that a person that, by himself, has such lackluster performance across all points of the game is still somehow capable of being called anything better than mediocre.