Master Yi cannot be slowed while his Ultimate is active.
Viktor's Gravity Field spell causes all units in its AOE to receive a stacking slow debuff and will stun any hero that receives 3 stacks of this debuff.
If Master Yi is within the Gravity Field long enough that he would have been stunned if his ultimate wasn't preventing him from being slowed, will he still be stunned?
Best Answer
Because Master Yi cannot be slowed, he does not receive the debuff or any stacks, preventing him from being stunned.
http://na.leagueoflegends.com/board/showthread.php?t=2052671