their doors are generally very noisy which means going to toilet at night will wake others up or alert them to your presence
they get very hot in summer (and even those with air conditioners that can be run without the car engine running will need adequate batteries and electricity generators which will not be popular at most camp sites)
they can still suffer major storm damage if there are large hailstones (or extremely strong winds such as a cyclone or tornado - but then most shelters would be at risk)
they are expensive to repair
they will not guarantee protection from spiders, ants, mice, bats, mosquitoes, etc - and these can be much harder to find and remove than in a small tent
they lack amenity, especially in prolonged rain events, although an awning may help address this if it is not windy
you will lose your campsite if you need to drive somewhere (eg. into town to get supplies)
even if it has a toilet, most will prefer to use a public toilet rather than manage the cleaning and maintenance of the toilet
you don't really want to be cooking inside a campervan or caravan long term as the steam and oils will affect the bedding, etc.