Lol... Bringing up Garen and Jax.... People have issues with Garen because they don't understand his regen. All of his regen comes from his passive which you should keep down at all times by poking him and exchanging blows oftenly. Jax can only counter so hard. He has his ability to dodge basics which does delay Riven's damage, but that's why you don't ulti and charge at a Jax like an idiot, you get him to pop his E, then you press W and R and beat his face in while he's practically defenseless. Yes, he has armor and can gain more by using his ulti, however this is why Black Cleaver was changed and why ArP was changed. Armor can be countered HARD because of how it works now, ask Talon and Darius if Jax is a truly a problem and they'll tell you his ulti ain't worth **** anymore.
If you're REALLY having issues with your ulti, learn how to time it... You shouldn't use it pre-initiation and you shouldn't be leading the pack since you don't have a gap closer. When using a bruiser that lacks a gap closer, you need to have a jungle that can supply the initiation like Malph, Ali, Xin, Amumu, etc. and then after they're in there, you get in. If they're disengaging, don't waste your ulti and give chase, it's a waste of your ulti and your time, just back off unless your team is keeping them slowed and/or CC'd, then you can pop your ulti and destroy them. Riven is an aggressive champ that requires some timing, much like Katarina.