Katarina has the highest banrate in the game at 74%, and now she is going to have a 15 sec CD ult

and she also had her rank 1 and 2 ult gutted, you kinda failed to mention that.

Katarina needs to snowball, and this dampers the amount of windows she gets naturally and will have to rely on her passive to get more.

Late game the CD means nothing really, everyone is grouped up and one teamfight decides most of the game.

Those stats are for plat and higher (which is only about the top 8%). Her overall number is 31.