They have to be modified to go faster than 65MPH, and a well tuned Unimog will rarely get better than 8MPG. They're military vehicles, so like the US HWWMV and M35A2 Deuce and a Half you'll be redlining to get them to 65MPH. Most all unimogs get that mileage and that top speed for the very reason you mentioned, dedicatied offroad travel.

The Frankenfinder is in just about the same boat, it may be a little bit more road friendly but I'd recommend against driving a custom SAS job as a daily driver unless its a super high quality professional job, not some backyard welded fab job.

If you want a reliable, solid axle, daily driver at a decent price get a used Jeep Grand Cherokee. I have yet to see an Nissan SAS job that I'd trust with as much highway miles as my Xterra will get (and I offroad alot).

Bottom line:
A solid axle that's been designed by a team of professional engineers who design suspension systems for a living is likely to be much safer and more reliable than one welded by your buddy who wanted to use his new plasma welder.

But if you have a dedcated trail rig then by all means SAS it, trim the fenders, lift it sky high, and do a engine swap.
Just don't drive it cross-country to attend GONE unless its on a trailer; and bring lots of spare parts for trail breakage.