Yi sucks if you stun him (even moreso than other carries, since he has no crowd control). Yi can easily be countered with an executioner's calling, due to the fact almost every yi relies on lifesteal to stay alive during his ultimate (he's a squishy, squishy dude.)
Yes, if you go pubstomp against teams without a good stun, he's going to win. But early game he can't do much against the people he slaughters later, and early and lategame, a well built caster with a stun can OHKO him, if he/she has a bit of team support.