To be clear, I don't mean this thread to be advice. I'm just talking about how I feel about this particular subject: in the context of the 5E variants out there, and more broadly 3PP material, I, as the kind of GM I am, do not care what the players decide to play.
I used to. I used to agonize over balance. Then, one day, I realized it doesn't matter. Most of the time I am building factions, enemies and encounters that make sense in the fiction, so who and what the PCs are is irrelevant to that. Other times, I am building encounters keyed to the PCs, which means I care about what they can do, but not where it came from.
I don't want to make it sound like I have no problem with completely broken character builds. Rather, that's a problem that can happen in limited,official only campaigns because, frankly, 5E isn't particularly well balanced to begin with and just in the PHB there are tons of broken build options. Again, if a player wants to break the game, THAT is the problem.