I read all the Cadfael books back in the late 80s/early 90s and loved them, but shelf space meant they got given to a charity shop. Then 3 years ago I felt nostalgic for them, and I've gradually reacquired and read all but one of them, and I've loved them all over again! They're like comfort food -- easy to read and digest, but still satisfying.
I recall reading a review of one of the TV programmes, and the reviewer picked out a specific phrase uttered by one of the characters as being unnatural/weird/clumsy -- and I went back to the book and there it was. But I didn't have any problem with it on the page, even if on the screen it came over as a bit clunky. Her characters don't talk as we would do, but I never saw that as a problem, nor did I find any stiltedness in it. In fact, later in the series there's a letter which is read out which is very formal and stiff, very different from the dialogue, and that reads as authentic for the time for the written word, which surely must reflect somewhat on how they spoke.
I wouldn't say her prose is dense in the Cadfael books -- in fact, compared to some of her modern detective novels I've read it's a model of lucidity! The only thing that does get on my nerves now -- which I never noticed back when I first read them -- is the odd places she puts the dialogue tags and the excessive use of exclamation marks!