For me there's a great quote from Betrayer
After thirty thousand years, warfare had come full circle. The sheer scale of humanity's conflicts disregarded the corrupt reliance on automation as seen in the Dark Age of Technology. Mankind was back down to swords beating against shields and men entrenched with their rifles, where the gods of myth were titan war machines and baneblade tanks. In his calmer moments, Kharn felt honoured. He was living through a second age of legend.
Essentially the conflict in
40k is more mythological and epic rather then pragmatic and scientific. There are moments of "current day" thinking (using the navy to full effect, infiltrating explosives, information warfare etc...)
and when in doubt...rule of cool