I feel that while the oath between a Knight and a Lord is rather loose, since it is purely for gold, the oath between a Lord and Duke is of much stronger stuff, that while not unbreakable, will make you some enemies if you break it without good reason.
I have also witnessed certain dukes able to solidify the loyalty of the knights of his lords, so that is something to keep in mind.
So my observations have been that loyalty is earned, not something to be taken for granted, in Battlemaster.
