knowledge of military hardware;
the difficulties of pre-technological travel;
the inadequacies of ancient or medieval medicine;
the complexities of urban civilizations;
I agree. It sounds as though hf of this sort would need to be called "hard hf" to differentiate it from mere "sword and sorcery" just as "hard sf" has to be differentiated from "space opera." In hard hf, the realism might overshadow the supernaturalism?
I got into reading Poul Anderson because of his sf, then read anything else by him, but this does not make me a fan of hf as such. I have read Anderson and some of Tolkien and Moorcock and that is about it - although see below. I dislike Tolkien's anti-Darwinism. The various kinds of beings, Elves, Dwarves etc, merely "awoke." How? What was their physical origin?
A while back, a guy called Art told me that he was helping a local fantasy writer to draw maps of a fictitious world. A while after that, a bunch of guys in a Lancaster pub, including a Larry and a Colin, were arguing about the Spanish Civil War. After the argument, Larry told me that I should ask Colin about his books, adding that he wrote the "Farlander" series. As a matter of fact, Farlander is Volume I of the Heart Of The World series.
Colin (Col Buchanan) took from his rucksack hardback copies of Volumes I and II, all that had been published so far, signed them and gave me them. Of course, I read both. I think that this series might count as "hard hf." Its fantasy element is indeed subordinate to its social and military realism. Col writes about warfare as if he had been in the thick of it. And these volumes read not like their author's first and second novels but more like the works of an already established writer.
Col left Lancaster for the West of Ireland to work on Volume III. (And I still do not understand what went down in the Spanish Civil War.)