The only thing I don’t like about Japan building factories on the mainland is if the game is going good for Japan, and once they eliminate China and India and any Russian threat, those factories become almost useless because Japan now has to branch out to sea.
Yeah, you can still use them, like to build transports to load up the existing land troops, but most of the naval production seems to happen at Japan itself. Plus, unless your navy is spread out, then you will have defenseless transports all along the coast. Then again, I guess if it is going that well for Japan then they can afford to “throw away” a few transports in order to get men on the islands or on Australia.