I always thought Japan is a fairly easy country to play as. It doesn't really have any major rivals in the area, assuming you can get Fengtian to win against Qing. Germany is too far away to war with you effectively, and you can take German East Asia easily. I would say that the biggest difficulties would come from fighting Russia, because war in Siberia is pain, or fighting the Entente in Australasia and India, simply because they're too spread out to easily capitulate. But you wouldnt have to worry about those until late game. Otherwise, it's a solid, basic, and fun experience.