It's the shipping
In my experience....and for what its worth...(it's free), the Mercedes trucks including Unimog's are used in most, if not all ground based military units in the world. As a result the parts are already in most parts of the world. The problem is that electronic parts can't normally clear customs. Sometime its the oddest things that give a problem to customs and automotive parts are one of them for whatever reason. If the part is an electronic part FORGET it. If its a part that's already been approved and cleared it's a snap, any dealer can get the part in but if it's not a part used and previously cleared into the country it can get tough. That’s part of the value of the Mercedes brand in my thinking.
As far as the Ford versus Mercedes my thoughts go back to the earlier comment about how high the vehicle is. Equipped with the MPT 81 tires the Ford is pushing 11 feet. GXV Unimog based bodies are normally just over 12 feet with a six-foot seven interior height. I think most Unicat's are about the same. For my money, a foot difference in overall height is not even a consideration. The Unimog can take a side hill or an approach that a Ford could not even consider.
Now don't get me wrong the new Fords are much better than last year's however, there is a reason that new Unimog chassis can cost almost 4 times what a new Ford F550 will cost. From the 33,000 GVW to the 22" in ground clearance not to mention the worldwide serviceability and 1,000,000 mile possibility plus everything else I can think of....there is really just no comparison. If I was only staying domestic, the Ford is fine and makes a good camper. However if crossing the border's are in your sights then I go back to "there is no comparison". A Mercedes-Benz, MAN, DAF, or any number of globally used chassis made a far better choice in a true Expedition Vehicle.
I'm sure there are people out there that have a different opinion, and this is only my opinion. However if I was going to make an investment based on opinions then I would want to see the passports of the people making the opinions.