We've actually balanced monopoly bonuses thrice. And the last time that was forced by a change on food, farms and happiness needs.
That's not to say one luxury can't be imbalanced, but even if that's the case, there are plenty of mechanics in the game to handle them.
I'm finding plantation luxuries on forest the hardest ones nowadays, usually citrus, for the difficulty of connecting them. Sugar on marsh has always been difficult, but sugar may spawn at other tiles without features, citrus on the other hand rarely spawns in grasslands. At least for me.
But forest have their own reward, it's easier to have camping resources which are very easy to connect, and makes herbalists more profitable. So, maybe citrus is balanced anyways.