well positive is that submarines become navy's equivalent of "air" in that only weapons that are designed to attack them can hit them, instead of anything with big enough aoe, similar to how air units are shot by aa only. (I know air can get hit by non aa like artillery but thats something you cant even micro to force unlike groundfiring submarines, and mostly happens accidentally and when someone flies a swarm of planes over a mavor or something as it is about to shoot.)
it also solves what this thread was originally about, which is atlantis being useless by it not getting easily destroyed by battleships groundfiring it while its underwater, and every other submarine, it also turns them OP since now they arent countered by this bug, and thus have to be balanced around weapons that can hit them instead of groundfired surface weapons.