very expensive to buy and repair
big and heavy and significant wind resistance when driving
more petrol usage ~10% more fuel
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
you have to climb down the ladder to have a pee at night - a recipe for disaster!
snakes like to hide under cars in hot weather - you will not see them if you are getting down from a roof top tent instead of walking around on the ground
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, harder to find high enough wind breaks, 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 rooftop tent is designed to carry things as well but then you will probably exceed the weight limit of the roof
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
does not provide much amenity during prolonged rain periods - you need to bring a separate tent for that
when you go for a drive, you will need pack up and leave something to keep your camp site - or lose it - you need a separate tent for that too
you still need a separate hiking tent if you decide to do an overnight hike
you may not realise there is a major overnight flood occurring until the water is up to your tent level and then you may need a helicopter out like the couple camping at
Newbridge Recreation Reserve camp ground needed when the river level rose dramatically overnight
many campgrounds now have bollards which means the vehicle may need to be near a noisy or dusty road and you will not be able to escape to a quieter section of the camp ground
some designs require two people to set up and pack up
if you get injured camping, get a sudden attack of sciatica, or have mobility issues, you may not be able to get up or down the ladder