I haven't had any problems with concealed or hidden battlefield units; I experimented with hiding all my army (except the Taisho, of course) and sat around for a long time while the enemy army marched all over the map looking for me.
My prob is on the strategic map. The AI always seems to know where to hit with it's ninjas and other hidden agents, to the point where I always use the .matteoetc. cheat to be on even footing.