Ult CD or some of her damage. She beats a huge chunk of the cast in any lane, and really is only balanced by the fact that she's easy to gank. Also really nothing is more annoying than having an enemy fed ADC and this Kayle that did nothing but feed all game but hurr press r can't touch him.
To be fair, it's 2 second of immunity. A lot of characters can compete with that via stuns, huge increases to survivability, etc... --- I suppose I should probably update this signature soon.