Lets clear my SS statement up a little bit:
SS are immune to Japanese aircraft unless a DD is present. Japan starts with a massive amount of them. +1 US.
SS cannot hit Japanese Aircraft in any situation. This forces Japan to take naval hits which are something Japan never wants to do. +1 US.
Japan cannot expand without naval supremacy. Each ship you eliminate from the Japanese navy lessens their ability to expand confidently. Packs of SS accomplish this efficiently as you don’t have to commit significant resources in capital ships + aircraft to accomplish the same feat and fear a counter-attack that will sink your fleet too. +1 US.
To defend against SS special abilities, a DD is required, making SS hits more valuable in any given attack round when no DD is present. This confounds a Japanese defender because a choice has to be made between preserving a DD to prevent this or losing a more valuable ship than an SS (which is the cheapest to purchase). +1 US.
In order to nullify the SS attack advantage, DD have to be purchased at 8 IPC. As SS cost 6 IPC, each trade of an SS for a DD nets the US +2 in economic value, an economic win for the US. This becomes even more valuable as you start to chew through Japanese fleets that do not have DD present. Even a tipped BB or CV is worth it if Japan has to fear a second strike from those SS after you submerge them when the DD gets eliminated. +1 US.
SS present Convoy problems once all DD have been eliminated from the Pacific. Each purchase of an 8 IPC unit to stop a convoy of 3-4 IPC is an economic benefit to the Allies because you take part in a resource denial campaign that prevents Japan from spending resources to expand beyond places it already obtained. Even with trading a 6 IPC SS for the US, it is FAR more efficient than the resources required to take a DEI from Japan, defend the fleet accompanying the landing and then holding it. +1 US.
The only true disadvantage is that SS without a capital fleet is incredibly vulnerable to DD + air strikes. Of course, if Japan is attacking SS with DD and aircraft, they aren’t using them to expand their economy. I’d accept a break-even here, but I think the advantage is still +1 US for hindering Japanese expansion.
Overall, SS is king in the Pacific, particularly in roving packs of 4-6 SS (I buy maximum SS for the pacific the moment the US can join the war) where you’re more than willing to trade 2-3 SS in order to sink a couple ships Japan will have to replace when your purchase round includes 4-6 replacement SS off of W.USA.