Donkeys are good little guards. They don't have as much of a flight reflex. They stand and think it through. This is why they make such good guard animals, because instead of running they attack. Mini's will not stand up to brush wolves though; they will surely be eaten.
They aren't like horses. Well, actually, they are like a horse, but more so if you get what I mean. You can't force them to do anything and anything you train them to do will have to be 'their' idea to begin with.
All in all, donkeys are very good pets, but they love you like a cat, not a dog. I personally love cats so donkeys are great.