Donkeys are typically quite a bit more well endowed than most horses are. Especially in the length department. This can pose some challenges if you’re not prepared for taking that kind of depth, though some donkey cocks will bend a bit if there is enough resistance. Personally, I find stallions much more enjoyable to be bred by than jacks, but I have had a few male donkey lovers that were quite the gentlemen in the breeding shed.
When it comes to mules it depends on which trait is more prevalent, the horse, or the donkey. I’ve met some mules that were hung like stallions, and some that were hung like jacks. The only mule I’ve ever been bred by was more horse like in that area, so he was quite fun.