I would think the transmission would be under less stress on the road, getting up to temperature and being under a fairly constant load. Much less than going backwards and forwards all day , sometimes 24hrs a day loading a hopper or trucks, or doing a load and carry operation. Having moved on the road many different types of wheeled loader and motor scrapers by road that is my opinion. Clark transmissions as used in Michigan's were pretty bullet proof, I found Allison transmissions less reliable. I would have been worried more by the use of one driver for such a trip, followed by tyres. That was an epic trip by anyone's standards, he must have had a heart like a Lion.