I don't know much about mules, which is why I'm fascinated with Sheba the Breyer Horse/Mule. If you are also mule-ignorant, you might want to go to Lucky Three Ranch for their interesting and well organized information about mules.
I especially loved the section What are the ass' mental characteristics? (For all of us who thought we knew.)
For instance, I didn't realize that mules are more difficult to train than horses, according to Julie Kennedy. Or that they're stronger and therefore, harder to coerce into loading or other situations where you might be able to out-maneuver a horse.
More to come on Sheba.