I'm confused why people truly think she's broken, I mean yes she has a powerful mid game and a decent early game now. Her Late she is strong but can be shut down by literally anything that is hard cc. This relates to her win rates on op.gg she may have the highest win rate pre-25 minutes but it drastically falls over the next 10-15 minutes, and before someone says something about her being number pre-25 just think that some champ has to be number 1, its a list that's how it works.
Realistically this means theres a great way to deal with her, control her early game, either by an anti assassin mid laner like Lissandra, vladimir, malzahar, etc. and/or have a jungler camp her so she cant roam easily. Shut down her early game and she won't be able to come back, and if she does happen to get out of hand she'll fall off as long as the enemy team has good peel and tanks.
Lastly what's up with people thinking mana is her problem? she's gonna be able to do damage with or without a mana bar, and her shtick is a very reset heavy playstyle so adding mana could even ruin her initial design, which to someone who wants her to be unplayable I guess is the right idea but from a fair design point that's too harsh, it would be like removing Jigglypuffs jumps in melee because everyone else only gets 1 extra jump, unless there is more compensation then just adding a crutch to her its just not right. Kat doesn't need mana to be balanced, IMO she's pretty balanced as is, because she works around her cooldowns instead of mana. She has to get a kill/assist to get resets so if she just jumps in a blows her load but no one dies, now she's screwed thats how her kit was designed.