they are much more expensive and tend to depreciate in value
more can go wrong - gas leaks, water leaks, towing issues, wheel issues, van brake failure due to failed connections, etc
unlike tent scenarios where worst case you just buy another tent, stove or fridge, if things go wrong with a van, they can be time consuming and expensive to fix - if you can fix them - parts or mechanics may not be easy to access - especially over holiday periods or in more remote areas
need a fairly large area of reasonably flat ground plus use of ramps under wheels to ensure relatively flat
high risk of accidents whilst being towed, including loss of control at speed, hitting pedestrians due to poor visibility, or poles when cornering due to poor judgement or visibility
they will require use of more fuel to tow
some do take some time to set up and pack down
they can get very hot in summer time if there is no air conditioning
they can get very cold in winter unless you add extra heating such as a diesel heater but this can be noisy for other campers
some require considerable physical abilities to set up and pack down, and some require a 2nd person to assist
require storage when not in use
constant risk of them being stolen by being towed away by thieves when unattended
requires an appropriate tow vehicle for the load