The most unbelievable part of Star Trek combat of any kind was The Red Shirt effect.
In real combat, death doesn’t discriminate. Anyone who steps into battle tends to have a roughly even chance of dying. So when DS9 got invaded or the Defiant took a pounding, we should nearly always see one or more primary characters biting the dust.
And while that did happen at times, like with Jadzia Dax, it didn’t happen nearly as often as it should have.