A Sub vs a single destroyer could go either way, all depended on the firing angle and if the destroyers were alerted or not.
A sub vs 2 destroyers was dead, just a question of time. Because one destroyer could lay off to the side, pinging it with sonar, and tell the other destroyer when to drop depth charges.
Subs generally had one chance to take out a destroyer, and after that would have to dive deep and try to evade.