As I think has been touched on, the fluff is wildly inconsistent. The best that can be reasonably hoped for may be that the broad themes - Space Marines are super warriors and therefore > humans all-round; stronger and tougher than Eldar; more disciplined and better trained than Orks etc. just to take the most popular faction.
The point after that is, is the game enjoyable in itself and satisfying.
If you take movie marines as an\example, as was said marine-on-marine combat is a chore. It also involves one person playing a skirmish game whilst a non-marine plays 'mass combat'(ish). Both will get old. One is also not necessarily satisfying from a hobby perspective and won't make GW a lot of money....!