A mule will work hard for ten years - for an opportunity to kick you ONCE!
Only if you abuse them. They do hold a grudge and have a memory like an elephant. If the want to kick you they won't miss. Watch a bunch in herd dynamics. Very seldom do they kick each other. Some times they will throw a foot out as a warning, but not touch you. My wife had a mule that would do that when any man walked past.
I have only had two mules out of 40-50 that would kick. One was 7-8 years old and came out of a herd of 75 that had not been touched except castrated and branded. The other was one that had been treated rough when some one started her on shoeing. She would hurt you bad.
I don't know if they can actually see their back feet when they put them down, but they sure never put them down where they should not. Walking,their backs usually are put down pretty close to where there fronts landed.