These are the "main" issues in my opinion for SS's. 

1. Torpedoes are WAY to slow. I think the Torpedo speed could use a slight buff, trying to kill a DD with torps is a utter waste of time. I understand DD's are supposed to be a counter to SS's but for fuck sakes it should be at least possible for a SS to kill a DD. I fire torps and they either dud because I am to close or the DD outruns the torps.  

2. SS's can't overheat underwater. Really? That is a complete joke. 

3. SS's are very slow. I don't know if this is an issue because of the lack of overheat when underwater but either make them faster or allow overheat underwater.

4.  Visibility. The visibility is garbage but in honestly I would live with the visibility staying the same if they buffed the speed. 

I have all the SS 1's and like 20 games away from the UK SS2 (or SS3 because it skips a tier, don't know how that works) and so far they all have these issues.