I’ve taken Russia on J4 and on G3 before. But it’s ultra rare and usually against someone who decides to buy tanks on Russia 1 or even worse, I’ve seen a 2 fighter build on Russia 1 (classic.)
But they have it right, in classic AND revised Japan is pretty immune to American aggression for a couple rounds at least, maybe even 3 rounds so clearing islands is easy. The bad news, cleared islands are easily conquered. Which is why, normally, in a KJF situation I like to leave 1 US Infantry, 1 Armor, or 2 Infantry on conquered islands with value on them. (New Guinea, Philippines, Borneo, East Indies, Philippines, Okinawa, Hawaii.) That way if I loose the fleet somehow or Germany sneaks out they cannot liberate my islands.