Riven is a snowball champ, basically, she is a hit or miss pub stomping toon that focuses on player's facerolling in the correct spell order at the right pace. She snowballs extremely well and is quick to penalise the enemy laner with even the slightest edge ahead. That being said, the opposite is also true, so if she is behind, you're done.
Lee Sin is a problem child not because he is currently thriving or anything, no he isn't, at least not now (all thanks to Cinderhulk). His issue is a combination of having a kit that is massively overloaded to be able to basically achieve anything whenever and however he wanted, forgiving to player mistakes to the extreme, while being overly protected by the over-bearing Riot, thus spoiling him. When the next season comes around, I have no doubt he would simply rise back to the god tier status he had hold for the past few years, because his kit is just so much of a mess he is never bad in anything hence if he is good, then he is just too good.
They both have their own learning curve attached to them, so you can make wonders by mastering them. Difference is, Riven has an actual weakness in her kit, allowing her to be counterplayed, while Lee Sin fundamentally doesn't have one for whatever reason. You need to completely rely on the fact his player sucks or the balance is currently against him to properly counter him.
Either way, take your pick.
is a bullshit champ are either in too low an elo or have never played 
, 
have huge mastery curves.
