To settle the debate, Riven is not overpowered.
There's a difference between being overpowered and broken. Riven is not overpowered, she is broken. The reason she is strong is not due to her scaling, her shield, or her mobility, it is her animation cancelling. Riot has already stated that the animation cancelling was not intended. I personally think Riven is in a good spot, sure she can win lane pretty easily, but Tryndamere can win lane with less skill, same with Darius, they simply just out trade you. Riven is weak when it comes to late game, because 1 stun and she gets blown up. It doesn't matter how much damage she does, she's building glass cannon, that's why she does damage. Look at Ekko, he can go full tank and deal more damage than half of the assassins currently in the game.
TL ; DR: Riven isn't broken you just suck.