Well, novels are novels. Marines mounting multilasers on tamed Tyranids, 16-year old Celestians acting as elite bodyguards, and Battle Sisters playing cards and getting drunk because it's fun. Part of why, personally, I deem licensee products not as "reliable" as studio material. Apart from some novels actively contradicting each other.
In the end, everyone will have to draw the conclusions for himself. Given the ambiguity of the sources and GW's reluctance to officially take a stance, it comes down largely to a question of personal preferences. Some players will want Marines to be "well equipped" popular superhero studs, others prefer the abhuman indoctrinated machine of war approach. It depends on how gritty and grimdark you want the setting to be.




