If you look at the map at the start of the war Italy has 8 of its 10 IPC sitting on convoy routes. IF Italy takes Southern France/Normandy/Greece and Yugo. All those territories are sitting on convoy routes. This is a serious weakness/issue for Italy and I think the Allies should do everything in their power to exploit that.
Over in Japan 25 of their starting 26 IPC are all sitting on convoy routes. FIC/Malaya/Philip and the Money Islands are all sitting on convoy routes. Once again this is a weakness/issue for Japan and the Allies should go after that weakness.
Italy and Japan have the exact same problem. If the Allies are going after this weakness they have to do something they do not really want to do, build navy. Which in turn helps the Allies as Japan/italy have less IPC resources devoted to land/air power.
So, sub warfare is out of whack in the sense that it is the Allies who are the ones hitting the convoy routes and strangling the Axis instead of the impression of WWII was it was the Axis pounding the convoy routes and strangling the Allies.
Now one could ask this question: Great point BUT if the USA and to small extent ANZC/UK are building some subs. Are the Allies in the same situation of putting a lot of IPC into subs and not land/Air? Well, yes they are. Then again though subs can also attack. IF lets say USA has 20 Subs all along the coast of China in various stacks. Japan navy pushes all in some where on the map and has a huge show down with the USA fleet. Well, all those subs can come off convoy attacks and attack en mass on the Japan navy for 1 turn. If the USA clears the pacific of all Japan Warships, well, Japan is on the ropes and ready to be knocked out of the war.
Then subs go back to hitting convoy routes. So, Subs are dual purpose unlike lets say a Dest/Cruiser/battleship. Once the enemy fleets are gone then those naval ships have no real purpose any more except for the occasional one turn of shore bombardment here or there. So, you could argue surface ships are wasted IPC in the end game.