The units that come in a van utilize the van's engine to operate a very simple machine. They are sold as more reliable due to the simplicity of the design. While reliability can be argued and is very subjective, the truth is they are simple and easier to work on and diagnose. Their biggest downsides are higher fuel consumption, being reliant on the van to operate, and less performance than some of the larger slide ins. The performance issue is due to the use of a weak magnetic clutch used to drive the machine with exception to the
Vortex and
Aerotech. My opinion is that the clutch is expendable, they generally run less than $300 and can be installed quickly and easily.
The "slide in" units that bolt in have their own engine, and engine accessories. Some are still relatively simple while some are very complex. More parts means more opportunities for problems. Reliability with these machines varies from manufacturer to manufacturer and unit to unit as well as maintenance on your end.
There are many options and not one of them is the last word in machines.