In the case of the larger Clayton units the price tag is nearly double the cost of my diy seat base battery, 2 KW inverter and twin solar panel set up.
I'll admit the Clayton is prettier and can be used away from the van but by the time you've got one in the van charging from the vans alternator and rooftop solar it's just a more expensive way of adding 230 volt AC to the van.
Versatility is if you want to use the power station for other uses too and fair enough it's hard to take a Transporter everywhere.