The problem for Yas is that Riven won't make it a trade, she'll take his aggression as a sign to go all in. Normally, Riven has the problem that she'll need to expend a q charge or two to get in range for anything, which blunts her trade power. If yas runs up to try to trade, riven will just instantly w, auto, q, auto, q, auto, q, auto, dash. A shield may mitigate 1 auto, but a stun and knock back will give her 2 free autos, plus basically the damage of an auto on every q. Yas should never win that matchup against a riven that can do basic spell weaving.