I was a big fan of roof-top tents (and still am!) but they do have their numerous disadvantages.
I like the fact that they are elevated. I don't necessarily think it's much safer, but they give you more privacy I think. You feel a bit separated.
It provides an always flat surface, with a mattress which is a great bonus compared to a ground tent. Also, the ground doesn't need to be level, you can just use a log or your maxtrax to level the vehicle. Then you're all set to open your RTT.
As many disadvantages they have, there are some butsolutions to these. However, that also means spending more money.