Spare tire on roof:
Looks great on a 'Mall Crawler' or an urban 'Bug out rig'. In practice, I can tell you first hand, that dealing with a heavy tire/wheel the size you'd put on a lifted 4x4 van, lifting it over your head to put it up, even getting it down off the roof, is a real PITA. My back hurts whenever I recall the experience.
Then there's also long term sun damage, like you mentioned.
I ginned up a temporary mount up front, until I finish my rear bumper and 'swing aways' project.
1995 E350 7.3 Diesel, 4x4 high roof camper, UJOR 4" lift