So, you take a totally green animal, throw a harness on, hook up logs or whatever and start working? And things go well? Nothing broken? The beast automatically understands and all is well?
Here's what happened with my saddle horse and out next door neighbor who was a horse trader/shoer/whisperer. He hooked her up to a sled because he'd been told she was "harness broke". She immediately started bucking and acting up. The sled moved a little bit, the harness broke from all the jerking. She was a saddle horse and not completely new to the world of bridles, reins and commands.
I'd say it takes a bit more than hooking them up and going to work. Maybe you have a special touch?
Plus, nobody has mentioned getting the animal conditioned. They get tired. They get tired faster if they are new to the work. They're like us in that way.
A logger kept his horse corralled at the logging site. The horse broke out and went home, which was quite a ways away. The horse could not work the next day because it was worn out from the trip home.
Horses have to be trained to the task. A horse that drag a calf in the arena would freak out if you hooked it to a cart. Draft horses are much calmer but still, if the draft horse had only pulled a cart it might spook if asked to ground drive logs. Also the horses have to get along with their partner since they work a foot away from each other. Needless to say their pulling abilities have to match. I can only assume mules are the same. And like I said I have no experience with mules (or oxen) but draft horses are heavy animals. Mules are much lighter so won't be able to pull or stop heavy loads.
BTW OP where are you from?
Maybe TUD will join in. He knows a lot about animals.