My ultimate BOV would be a Subaru Outback with a mountain bike on a rack. Outback is dependable, has decent storage, has 4WD, and is gas efficient. Mountain bike is good to have because it requires no gas, is quiet, and is very versatile.
The Unimog gets about 6 miles per gallon tops. In my book that makes it a poor bugout vehicle. Consider that gas mileage is your primary concern. When you run out of fuel, there probably isn't going to be any more, so the Unimog becomes a 5-ton flower planter.