its not that she's problematic, but that she is inherently imbalanced and lacks direction
she is still fun to play but they want to make her more of a champion who's performance develops simply to other champs
basically she does great early game and is super strong with mantras, but kinda falls off late game because mantra's will not be as strong as regular ults and she doesn't usually get more than one mantra off in a team fight,
secondly she has some heavy favored mantras (RQ ALL DAY ERRY DAY) and the other 2 are used relatively less in comparison, they want to open up more possibilities for other mantra uses while also giving her more opportunities to use mantra
she was imbalanced because she is stronger at early game and very heavily reliant on Q, they want to smooth out the distribution of power in her kit, to not be so reliant on q/RQ and give her a more healthy progression as the game goes on, instead of getting weaker