I think the authors were trying to make a nod at novel conventions, so I can overlook the trite mystery and romance aspects of it. As a few other people mentioned, the sex scenes seem out of place and a bit overboard, but what really got me was the way Fanny's reveal as a woman became such a big deal. She immediately starts wearing women's clothes around the house, and acting the lady somehow even though she has more than proven that such roles are based on nothing, and her romance with Jameson turns into a triumph of heterosexuality over his homosexual pining for Weston and his previous relationship with Ignatius. His preference for a "man's mind in a woman's body" couldn't help but resonate with me as somewhat heterosexist. Even though the characters are supposed to be of their own time, Jameson has already established himself as having had passionate same-sex affairs in the past, so his changing preferences seem to be a value judgment that hits too closely to the era the authors live in.
Additionally, while it tries, perhaps too hard, to portray African Americans in a sensitive way, it ends up exalting them in a way that feels a lot like anxious racial guilt. The book is clearly written by white people for white people. We're meant to be moved and captivated in the one scene that features more than one black person, which is a midnight burial, complete with musicality and spirituality. Then the rest of the book Ignatius goes back to being Sherlock Holmes. Gag.