American Mah Jongg is considered the most difficult style because only the hands listed on a card are considered valid. This requires players to memorize the card (or at least have a good grasp of it).
As such, during the late Qing reforms, mahjong became considered a social issue. Mahjong started to become outlawed indirectly as gambling and smoking opium became criminalized. The ban was not very successful, however, at least until the Chinese cultural revolution.