Master Yi is a hypercarry, meaning that if he gets fed he can absolutely smash an entire team, however if you cc him you could kill him due to his relative squishiness, furthermore, if you don't allow him to farm/get kills, he won't have the gold/xp necessary to be a huge problem. Since low elo players don't realize the potential or danger of Yi, I recommend banning him because losing just because your team dies to a single enemy lots