What to be skeptical of is when you see reports of (Insert Third World Country Here)'s air force in an exercise kicking the butts of American F-15s and F-16s, like in India a few months back.....those exercises are basically rigged, the US forces have their hands tied behind their backs - serves two purposes -1) The local third world country gets confidence and favorable pub 2) The Air Force, which is notoriously good at getting money, gets more ammo to ask for LOTS of F-22s, which is their main procurement goal.
In THIS case, however - believe it - a well-trained modern sub like this Swedish one is a very, very, very difficult opponent.
Really all modern subs from any country with a modicum of training is incredibly dangerous.
There actually were no "submarines" in World War II, other than the very late war German Electroboats, hydrogen peroxide subs, etc. - they were all "submersibles" - spent the vast majority of time on the surface.
Other than one shot in the Falklands, we've never really seen what subs can do now - and they've advanced faster than antisubmarine warfare advanced. In any large-scale naval war I suspect modern well-trained subs of any kind would rule the seas.
Fortunately, few of our potential opponents are likely as well-trained as the crew of the Gotland. It's very smart for the US Navy to lease those guys to practice agains (and I have a feeling the average Swedish seaman isn't violently protesting being stationed in San Diego - I wonder if they brought over their familes and put them up in San Diego, though.)
Do they have the "legs" to present a sustained threat in open water? Seems to me if they have to surface, even after a month, they become targets. And CERTAINLY they'll have to refuel, either from a surface ship or in port. Again, they become targets.
I can see how they would present a short-term threat to surface shipping...perhaps even warships. But over the longer term, nothing but targets.
Lets also hope that Sweeden isn't going goin to just sell these things to just any Tom, Dick, or Muhammad.