The only real major disadvantage is the need to resupply the crew. Unlike surface vessels, which can replenish at sea while underway, submarines must return to port to resupply.
Other than that, submarines have the advantage.
Enemy surface ships are the main target of submarine warfare
Germany hoped that submarine warfare would lead to war.
Anti submarine warfare
Germany's submarine warfare was unrestricted. These attacks led to World War I.
USA
submarine warfare
Submarine warfare has existed since the early 1900's, and continues to this day. Though the last ship sinking by a submarine was in the Falklands War, the evolving missions of submarines have taken submarine warfare well beyond just sinking ships and submarines.
unrestricted submarine warfare
Germany's unrestricted submarine warfare
Anti-Submarine Warfare.
it was Germany.
The Sea people