Originally Posted by Burning Rose
The fact of the matter is that most CCs have some kind of delay, and Katarina's ultimate does so much damage so quickly that it usually can finish someone off before you can interrupt it. Now Kat has all her CDs back, even if she only got an assist, and it's likely that someone else will die. BAM. Katarina has her ultimate back.
Besides, most champions with CC can't save their CC for Katarina's ultimate. Most of them are reliant on it to accomplish anything at all because of ability synergy(Veigar, Sion, Rammus, Pantheon, Warwick, Shen, Fiddlesticks), and a lot of them can't even do any good damage without blowing their CC (AP Sion, Annie, Kennen, Kassadin, Anivia, Cho'gath). If they wait for Katarina to ult, and Katarina waits for them to blow their CCs, do you know who usually wins? Katarina.
And then, of course, there's all the champions who can't stop Katarina's ult at all. "That's not many, and most of them are bad and shouldn't be played anyways". It's still a balance issue, and for your enjoyment, here:
Master Yi, Tryndamere, Ryze, Shaco, Morgana (I count her because of the delay on her ult - it can't really be used as an interrupt on a channel because by the time it goes off, the channel's done), Ezreal, Mundo, Sivir, Mordekaiser, Zilean, Teemo, Akali, Twitch, Gangplank, Karthus, Nunu, Nasus, Corki, Nidalee, Evelynn, Kayle, and for fun's sake, let's include Katarina herself.
So, the champions not included in any of my lists: Udyr, Taric, Ashe, Poppy, Alistar, Gragas, Jax, Soraka, Singed, Janna, Tristana, Blitzcrank, Heimerdinger, Garen, Amumu, Malphite, Twisted Fate.
Now, if you don't have one of those on your team, or GOD FORBID they use an ability on anyone other than Katarina? You're ****ed.
So yeah. "Just CC her" isn't really a valid reason for her to have the best early game, the best mid-game, and the best late-game of any champion in the game.