I think it is brutal/rewarding enough - not sure if we want subs to be sinking full squadrons of battleships in sneak port attacks. . .We have played a couple games with this rules now and while a rare tactic, it can be very rewarding. In the last game, a French sub snuck into Taranto harbor and sunk the only Italian transport, paralyzing North Africa operations for 3 turns. A sneak sub attack against damaged battleships and cruisers in port being repaired can be pretty devastating too.