This is what the developers have said on the issue: Maps and Minor Civs imbalance by region - #21 by tilanus
By that criteria, Mongols, Malay, and Bedouin would probably be okay since they could be used on many maps. Manchus and Koreans would be too limited because they wouldn’t make much sense outside of Manchuria or Korea. On top of that, Manchus are already a mercenary and Koreans could be a candidate for a playable faction instead.