I have campaign rules and guidelines... some of them change rules... these help the rules match the campaign. These sometimes include rulings on things like mulyi-class and even changes to spells.Do you do it?
Assuming a player comes along and his character is 100% by the rules, no Unearthed Arcana or any other playtest material, will you still shoot down a character design because you feel it is broken?
I am not sure if I would straight out say no. My first thought is that if they player can created something broken by some unusual combination of abilities through multiclassing then to play that character I would require a plausable backstory that explains exactly how this very unusual exploitive combination makes sense.
Would you just say no? Or would you allow it and then ramp up or tailor encounters to balance against said character.
So, no, I would not ban a character for power but might have already made rules that prevent its abuse.
The fact that a character is RAW legal Carrie's no weight for me when it comes to "is it allowed".