we host au pairs, and i actually find the opposite to be true. a lot of nannies expect a minimum of 40 hours a week (M-F) and don't like to work a split schedule.
the main limitation on AP hours is that they can only work up to 10 hours a day and 45 hours a week. they have to have 1.5 days off per week. other than that, you can schedule them however you want. obviously it's not the right solution for everyone, but it's pretty sweet for us! (and side note, we pay less for an au pair than we would for a nanny, even when you consider the extra food, a third car, insurance, etc.)