A while ago this very interesting book was published interviewing both mothers and Nannies about the difficulties and emotions toward each other and the children. One of the points in the book was that even though the Nanny take care of the kids, it's still the mom responsibility to direct her and be very presence in the children lives, some women prefer to hire a male nanny (Manny sounds really insulting to me) so that the children will have a male role model that's very presence in their lives.
Both male and female nannies can abuse the children or seduce one of the parents. I think that though we do hear horror stories, this is not the main thing to consider - there are also abusive parents and school teachers, it happens, and it's a risk one takes in leaving his kids with a half strangers.
I think that it's mostly about finding something your family can connect to and who is good at what he does and can work well within the boundaries of the family.