Well, neither of us have seen The 13th Warrior. But, as a mass generalization, I suspect that like most Michael Crichton books, the novel is probably very well researched, and the film probably jettisons nearly all of that.
I read a lot of Crichton novels when I was younger, but, most of the adaptations were pretty terrible.
Edit: turns out the novel is based on some pretty shaky research as well, which was later discredited. No idea on the swordsmanship, but I’ve got even less interest in watching it now.