User blog comment:PrayforDawn/What is Difficulty?/@comment-4846081-20140113002923/@comment-10767476-20140113142249

Clearly you haven't seen her win-rate, at LoLKing you can find that it is below 50% in Bronze and Silver, so she clearly is difficult to play.

I find Riven indeed very powerful, she has insane dmg and mobility, a stun, a knockup, and a shield. However, if you just run in and bash you'll die faster than you can kill anybody, squishiness (on which she scores 10/10) is certainly an important factor for difficulty.