very expensive
big and heavy and significant wind resistance when driving
more petrol usage
issue with roof capacity for your roof rack system
risk of it ripping off and damaging cars travelling behind
increases centre of gravity and risk of rollover
can be too heavy to constantly remove from or install onto the roof with only one person
you can't drive once you have it set up unless on a camper trailer - very restrictive!
you have to climb up a ladder
they are harder to close down as the top is high and you do need to use a bit of muscle to close them fully
you may need to deflate air mattresses when folding up (hence most use a foam mattress but then there may not be room for a quilt
you have to level the vehicle
must sleep where you park - this can be dangerous from other traffic hitting your vehicle at night or be very dusty if vehicles use that gravel road
take longer to dry out
can be very noisy in the wind
catch more wind as higher up and strong winds can kick up the bottom while you try to sleep
may have openings large enough for mice to get through
sun will hit you earlier and you get less sleep ins
can't use a roof rack for other things unless the tent is designed to carry things as well
choice of site may be restricted due to height clearance issues
generally colder in winter than a swag or 2 man tent
can result in condensation UNDER your mattress