The difference between Riven and Master Yi, skill-cap-wise

has a lot of mechanics to learn. She has a higher skill floor than a lot of champions (not by a ton though), and then she has a lot of mechanics and tricks that raise the skill cap, so there are a ton of levels of skill. Someone who has mastered Riven can use all of the tricks.

has 2 mechanics to learn. He has one of the lowest skill floors in league of legends (meaning anyone can play him at 75% capability mechanically), but the mechanics that he does have (alpha dodging and meditate resets) are so difficult to do that the skill floor and skill ceiling are miles apart, and are basically the only two levels of skill.

The other thing that separates the two is that has a fat-@$$ safety net and is much more all-or-nothing (which is the only other hard thing about Yi).