If Riven is so faceroll, why are 9/10 (being generous) Rivens I see absolutely terrible? If she is so easy to build, why do I see so many people building stupid stuff on her? If you look at leagueofstats, enough people build Ghostblade and Frozen Mallet on her that it's on her item list and her win rate is only 50.38%, which is incredibly low for how powerful a champ she is. If you want to say she's OP, fine, I agree, but don't act like all you do is smash Q, buy 6 Rabadon's Deathblades, and that's the extent of Riven.
The comparisons aren't very meaningful, either, as rage is bad in general on this map because there isn't a good way to build it and the only other real competition for manaless is Katarina (who has other serious problems and is considered bad even on SR) and Garen (who is also considered bad on SR and is weak late-game, which makes him even worse for Dominion). So, yes, when comparing her to bad champs, she is much better than them.