I don't have any experience with donkeys in general, but I do have fairly extensive experience with mules as driven animals. They mature at about the same rate as horses, though back in those days, people didn't concern themselves much with when the animal was actually mature. When they looked
grown, they were treated as mature animals and were generally expected to work on the same level as the adult. That generally happened around the age of 2-3.
The amount of dead weight that a donkey can carry is more than that of a horse, though, of course, it depends on the weight of the donkey. Supposedly, they can carry 30-35% of their weight in dead weight (though I've never packed with an animal so this is hearsay).
As for the stubborn streak of donkeys...LOL. It's not that they are stubborn, they are just exponentially smarter than a horse and they have to be trained differently. You cannot man-handle them the way you can a horse without ruining their attitude. You have to give them a good reason to do what you ask and "because I said so" is not good enough for them. I have always preferred horses because I don't understand how mules/donkeys think and they frustrate me easily LOL. However, once you get them broke and they trust you, they are incredibly willing and trustworthy. They don't panic and react immediately the way horses do, they think things through first.
Their training, if done correctly, doesn't take any longer than it does with a horse (though sometimes it is much faster). They also retain it very well. My Dad broke a team of driven mules when they were about 2 years old. After they were sent back to the owner, he sold them to someone who didn't know how to handle mules and he ended up having some very serious issues with them running off and generally being unsafe to drive. He ended up sending him back to my Dad and it took only part of one session for them to remember and revert back to the training that they had originally received.
There is a good reason that a good saddle mule is worth anywhere from 50-100% more than a horse with comparable training/temperament