Subs would be much easier to balance than CVs, who's game mechanic is just broken and never could be balanced. CVs will always either do too much or too less damage. And everyone who thinks, there has to be a point in the middle of to much and to less are ignoring the fact that there is a mm system. So it might be that you have a game, where they are right balanced, but the next game you are again to strong or to weak. And thats caused by game mechanics, which are so different to other ships ...
Subs on the other hand would use every game mechanic that is already in the game, just giv them low visibility and only torps to fire, very low hp, and implement a counter system on destroyers.
