you can't stand up and you need to crawl inside
vehicles tend to be cold at night (as they lose heat readily) and very hot during the day if it is sunny
condensation can be an issue as well so ventilation is usually needed - but with this one usually also needs flying insect protection unless it is winter in southern parts of Australia
the many windows also need covering for privacy and to reduce the morning sun waking you up too early
less luggage space for additional amenities unless the vehicle is designed for these amenities
you may have to leave your fridge at home!
where will you put your gear when its time to sleep?
consider a car roof pod to hold your gear
poor amenity and potentially unbearable when it is raining for long periods or when it is hot and sunny
you may accidentally flatten your car battery
poor connection with nature
no amenity to entertain guests
have to open and close vehicle doors at night to go to toilet - the van sliding doors are generally the worst and the noise from doors can really irritate other campers
rear hatch may not be able to be opened from inside