I need clarification on stuns, and pretty much every other CC in the game.

It seems to me that cc tends to be very unpredictable, and pretty much varies on a case by case basis. While a stun may completely stop a Nunu ult, Jhin ult, Zac, ect- - Kat or Wukong's ult are completely unaffected by stuns. Why is this? In the case of Wukong specifically, a stun acts more as a snare while R is being casted.

Stun: A unit that is stunned is unable to move, attack or cast abilities for the duration.

[I understand that casting the R means you're already channeling an ability, thus don't need movement,ability casting, ect to use it, however, as I mentioned before -- if this is the case, why do other champ's abilities get canceled during a stun?]

Feedback would be much appreciated, at the very least a list of interactions between characters and exceptions in CC mechanics would be incredibly useful.