I've only played a little bit of TT, but I understand from a balance perspective Space Marines have to be somewhat on par with the troops of other factions. However, from a fluff perspective, they seem to be so much more than just the equal of a Sister of Battle or Stormtrooper or what have you. These are posthumans who have spent centuries training, fighting, and studying tactics. They are armored head to toe in the best, most advanced protection available, and given weapons that can blow a man to smithereens with one shot - and that is just the average Space Marine.
Let's take the example of a Space Marine's ability to shoot ranged weapons. How does a regular person get really good at shooting? Practice, and probably quick reflexes and a good eye. Space Marines have had a lifetime of practice/field experience, inhumanly fast reflexes, and the eyes of an eagle.
Which is why it is hard for me to reconcile them into a game where they are expected to work with regular humans. While the Hierophant may be able to speak better or the Adeptus Mechanicum Adept could have a better grasp of technology, there are precious few unaltered humans who could even hope to match a Space Marine when it comes to combat or strength or toughness.
If a regular, non-genetically modified human in carapace and a hotshot lasgun is the equal to a Space Marine, why even have Space Marines in the first place? If they aren't the Angels of Death, what are they?