I didn't care for Rosalind Miles' first Guinevere novel and didn't read the others. She mixes time periods in a way that I suppose is allowable in a fantasy novel (isn't everything?), but I found it jarring. One moment we would be in a recognizably medieval world with knights in armor, etc., and the next we would be back in a sixth century pagan Celtic world. On the other hand, I like the fantasy elements in novels like Mary Stewart's The Crystal Cave, which are of limited scope and feel consistent with the belief structure of the historical world of the setting.
MLE, I take your point about Nissyen and Evnissyen, but they were individuals. Granted, individuals who represented unmitigated badness and goodness, but the kind of badness and goodness that individual people display. (I'm sure we can all list a few individual people, besides Evnissyen, who have caused disastrous wars.) That seems fundamentally different, to me, than a view of the cosmos itself as essentially divided between the forces of good and the forces of evil, with people perpetually in danger of slipping into the clutches of the vast forces of evil.