In the end it is a pure matter of taste. With a ground tent you only require a bit of storage space in the car and still have free space on the roof. Also you can build up a fixed camp. Rooftop tents are quickly build up and you always carry around your place to sleep. But still you need to be careful when driving at low clearances to not break your tent. This might happen on parking lots of shopping malls as well as when searching a nice and shady spot on your campsite.