The world-building issues were that Kushiel is a very rare and valuable sort of person, and this is marked by something (a red dart?) in one of her eyes. I find it impossible to believe that everyone would have forgotten about the type of person and the marker, though perhaps I'm applying unduly modern standards.

I don't get what you mean: there was actually a poem handed down through the ages to describe the mark. I only remember the last line, "pricks the eye of chosen mortals"; the other lines were about Kushiel and something about rod and weal and portals. Anyway, it wasn't forgotten, it just wasn't widely known.

The thing which was definitely unfair was being annoyed that Kushiel doesn't make sense as a masochist. Masochists have very definite preferences for the sort of pain they want, and Kushiel doesn't.

Kushiel was the Terre D'ange god of redemption through punishment; if you're talking about the main character of the Kushiel's Dart series, that would be Phèdre.

The point of the dart-mark in her eye was that she was blessed by Kushiel with essentially the ability to turn any pain into pleasure. She's not a normal human masochist, like the adepts of Valerian House, but rather an anguisette -- something that doesn't exist in real humans, so far as we know.

(For the most part, however, the character is written as a realistic human with strong masochistic and submissive desires, and only a few pivotal scenes in the novels actually require her to have pain-transforming abilities beyond those that a human could achieve with sufficient warmup. In truth, her gift seems to be the ability to instantly transform pain levels that human masochists have to "warm up" for with lesser quantities of the same type of pain.)

Anyway, the Kushiel's Legacy series is set in a parallel world to ours where gods and magic exist, so it doesn't seem especially egregious to have a character here or there with a supernatural talent, especially the main character.