[Herode]

I played a few test games with both sides but mainly Japan. If yuo play Japan ans the sea-lanes victory condition, you will easily win : the AI doesn't know how to deal with this threat. I guess you wil have a harder time trying to win with Japan if you choose another victoey condition, like Survival.

OTOH, the game will be more interesting if you play Allies. Japan is on the offensive side during the first turns (1942 at least), Japan has more landing forces than you have, naval and air initial superiority. You will have to fight. Not a superior AI, but at least a challenging one.

Of course, modding the scenarios is also an option.