It's because it's not reliable (Hit 3qs or get it off allies when you're solo laning) and the power it gives isn't that strong.
Really don't compare Riven's ult power to yasuo. Yasuo's isn't on demand nor does it give him and and he can't deal damage during it.
Not saying the CD is not too low, but that's why.
MagicFlyingLlama3 votesHours before Yasuo, the most broken champ in the game, gets nerfed, and people are already beginning to cry.
GET ON THE TRAIN, BRING POPCORN [slayer-pantheon-popcorn]
GULAG 4 U2 votesI'm not crying, I'm just pointing out why his ult cd is low. I'm also not sayinng it's okay.
Weathered1 votesHe can't do damage during it, however it does damage, and the enemy can't damage him. Saying that it isn't reliable isn't right either because you just need to use it with E and they can't dodge. The minion wave is an expressway to the enemy that they can't prevent except for very safe positioning.
GULAG 4 U1 votesYou can predict the E distance if you play yasuo and dodge it. It's hard but not impossible.
Also read the damage a second. 200 at rank one for an ult. It deals less damage than a single mage spell after level 6.