I'm not the best player, I'm average at best but i just want to share my opinion about subs:
Subs in Coop are really annoying. They just make it a really boring session of hide and seek and don't bring much to the game.
In ranked, as an average Cruiser main, it's just infuriating. When i get devstriked by a BB, I know it's because, I positioned myself incorretly, when I eat DD Torps, it's similar. But now i don't know, where the Torps will get form, I can't do anything about getting pinged, when my Damacon is on Cooldown and even if I see the Sub, I can't do anything as long as it's fully submerged. If I have depthcharges, I'm also out of luck because I'd have to charge into open water to use them, and then get focused by the enemy team. Dodging those torps is no option because they just change course and still kill me.
But my biggest problem is matchmaking. Yesterday I played a match witch 1 Cv, 2 Subs, 4 DDs and just one cruiser per team, and it was just hell. Cv spotted me, i got permapinged by subs and 3 different smokescreens spammed me with HE. It was just hell.
On the other hand most of the time, the subs are quite useless. They don't spot well and most players just sit in the middle of open water and wait for the DDs to spot for them so they can ping and torp. So they don`t contribute to the gameplay and the team.
All in all they just make the game extremly frustrating and i don't think that they have a place in the game at their current iteration.