I guess the point is to force you to fight submerged.
From the question you ask, I can tell you are also a deck gunner like me. The high ROF of allied surface ships is to compensate their otherwise stupid and predictable behavior and your deck-gun skill accumulated through multiple careers.
While this did not prevent you from engaging a armed surface target with your deck gun (I did it many times in GWX2.1, even sinking destoyers), it does forces you to think more like a real uboat skipper, so you are less likely to do so unless the condition is perfect (calm sea at dusk, evening, or night) and the range optimal (3000-4500 meters against AN armed ship).
And you probably shouldn't take on more than 1 armed vessel at a time.
Trust me, if it is too easy, you won't like it either.