Do you mean the roof tents that fold down to a shallow roof box and sit on top of the roof bars?
My mate had one that we used on top of his land rover. In summary, all of the pluses and minuses of a normal tent, but with a ladder…
He has an inflatable tubed tent now, if that means anything.
It was ok, don’t get me wrong, convenient to erect and collapse, but there’s no space to put anything outside the sleeping bit like your shoes/boots, and the ladder is a pain to negotiate when you find you need the loo in the middle of the night.
They’re pretty expensive compared to even a mid to top end tent as well.
If you don’t mean those, then please ignore! 👍
Edit: Ah, that’s what they call those tents that attach over the tailgate! They look ok if you’re going to use your vehicle as a sleeping compartment, but I’m struggling to see how they improve on a good family tent with bedrooms and a living/cooking area. I’ve not seen any at the campsites I’ve frequented the last few years.