I've been trying to get some value out of the Italian sub fleet by attempting port supply interdiction.
I noticed that I NEVER sunk a single supply point, while regularly losing a sub step, more or less every turn.
I ran some tests - Observations:
If a port has an air unit in range, I can NEVER sink any supplies with Italian subs. Probably ran about 15 turns testing this out with two subs sat 1 hex off Tobruk in raider mode. Italian subs lose more or less a step between them per turn.
If you remove the air unit, suddenly the subs start sinking a decent amount of supplies, often around 80% of port supply per turn.
So what we have here is something of a zero-sum game: If an air unit is present, you can sink nothing. If there's no air unit, you can sink a lot.
Of course the presence of an air unit should disrupt the sub ops, and cause sub losses, but not to the extent that the subs can no longer sink any traffic whatsoever?