@Savage Just remember to scale their effectiveness to the strength of their foes. They're obviously going to do better against human populations who've decided to reject the Imperial Truth than against, say, Orks as tall and beefy as they are.
Of course. If regular man could tangle effectively with the gigantic, monstrous, insane numbers of aliens that all would like nothing more than to use their limbs as pool cues then there would be no reason for us to exist in the first place.
I only jest, though Space Marines are super badasses. There is seldom a conflict that involves a Legion sustaining zero casualties and steamrolling their hapless victims.