As this novel is hardly aspiring to win a Nobel Prize of Literature, it is somewhat futile to remark too excessively upon its style and use of language. Briggs just wants to tell a nice story, and she does that a lot better than many, many other published authors in the fantasy genre.
Likewise, I won't comment excessively on her getting her facts wrong. (For example, the Arthurian Romance "Iweine"[sic], as referenced in this book, is not about a knight atoning for giving up adventuring after getting married. That would be "Erec", also by Hartmann von Aue. But as few readers of Briggs will have to write a test about medieval German romances - and those few who might should know better than getting their facts out of a fantasy novel - I think we should let it pass.) Nor will I say too much about her poor grasp of German. (Honestly, it sounds hilarious to native speakers.)
No, what *really* bugs me about these novels is the fact that their heroine is always supposed to be that fundamentally rebellious individualist, yet mostly acts, feels and thinks like the average All-American choirgirl.
Sorry, but just giving your heroine a tattoo and making her drive a rather unusual run-down car does not suffice. In every way that counts, Mercy is 100% conventional, very likely because she reflects the sensibilities of her creator. She even worries about nudity and sexuality - a lot - in spite of occasional allegations to the contrary. And of course, she attends church.
Now, don't get me wrong. It's not exactly what she *does* that disqualifies her for the role of rebellious misfit that she supposedly incorporates. You don't have to be a pot-smoking, kinky, polyamorous neo-pagan in order to be an individualist. Quite the contrary. But Mercy could easily fit into any white american suburb, once you get past her outward appearance.