Balance of Terror is considered one of the best TOS episodes.
But the battle seemed a bit one-sided. The Romulans were fleeing and had no decisive defenses of weapons. The mighty plasma weapon proved to be limited. They, their fuel running out and in despair, tried to shake the Enterprise off their tail with old bombs and debris.
The question to ask is if a U-boat with torpedoes and underwater "stealth" capabilities had a chance against a destroyer with much greater speed, sonar, torpedoes, depth charges and gun batteries. Did U-boats sink destroyers in WW2? Yes, they did
on occasion. Did they escape destroyers to fight another day? Yes, many times.