i find subs to be quite fickle. They serve a very niche purpose. Their lousy defense makes them, at best, cheap soaks. Their convoy ability gives them a higher util, and planes not being able to hit them without a destroyer is also an interesting ability to be capitalized on! I like them if the US needs to be convoying aggressively, I like them if japan is light on destroyers, i like them if italy has a tenuous hold on the Mediterranean. the abilty to sub/bomber strike with the US if they don’t have gibralter is nice. I only buy subs with the germans if i think i can keep the UK low on income for a few turns, i only buy them with the japanese if i need to soaks to maintain parity w/ the US in the pacific. Occasionally w/ ANZAC if i can get away with convoying spice money (but would rather just take the terrirtory). I can’t ever remember buying subs w/ the UK or USSR. I keep telling my opponents that if china gets a territory with sea access they immediately get sub pens, but they never believe me.