Took a black Freelander with a Myway roof tent to Morrocco, the misses give me a lot of grief about spending just short of a grand for what was going to be a one trip tent.
When we were in the forest the people sleeping in ground tents struggled to get pegs in the ground - saw our first scorpions - instantly forgiven for spending the money.
In the desert it was nigh on impossible to put up a ground tent when the wind was blowing, those with roof tents made it look easy, the hard shell ones easiest of all.
We took a portaloo and used the cutain section as a changing room / toilet tent / shower cubicle. We had been travelling sveral days before some of the group found out about the portaloo, we were accused of cheating, I know my misses, there is no way I would have convinced her to take a walk with a shovel.
For me anyone can be uncomfortable, comfort just takes a little planning. You're going to be fairly short to be able to use the extension as a shower tent in a Freelander.
The biggest medical problems the group faced were "food poisoning" several people were very ill after eating badly cooked omlettes, and "bowel/bladder problems", thought to be caused by holding on too long, so for us the portaloo was a good choice.
One of the things we did notice was the rooftent acted as a sun shield - a good idea in a black car in the desert.
On they way home through Spain it poured down for a couple of days, I got soaking wet packing the tent away, even when we needed to nip into town, but the contents stayed fairly dry.
At slower speeds(>50mph) the tent was hardly noticable and didn't seem to effect fuel comsumtion, at higher speeds(<70mph) it was noisey and she supped fuel.
I think these comments could be viewed as typical for any roof tent,I only have experience of the Myway, which did a stirling job for the trip. I sold it on ebay when we got back so inthe end we only lost £300.
All roof tents have the same drawback as motor homes, if you want to pop into town for supplies you have to pack everything up, the unpack again when you get back, and yes we are caravaners back home.