I don't think I could have read it as historical fantasy, because although I didn't know that much of the history I knew enough to know that it was based on history. (Also, I got spoilered for book 2 or 3 by walking past the possible side of the battle of Heavenfield and reading the information board there...)

For me, the main problem was the very modern attitudes to sex and relationships, which is a major bugbear of mine when it comes to historical fiction - Philippa Gregory does it too, and having tried one of hers I wouldn't read any of the others because of it.

(Oh, and the "irresponsible" thing about the abortifacient herbs is that these are real herbs. You could find a picture on Google and go out and pick them, and brew yourself a tisane. But actually, they aren't the easy and reliable solution that Griffiths depicts them as. People die of liver and kidney failure after dosing themselves with pennyroyal. Fantasy herbs would be fine, but I worry a little bit that this might give people a false idea of the safety of real herbs which actually aren't that safe.)