Jetwashers are fine for shifting mud off the underneath of your landrover but they're terrible for washing cars.
At best the spray gets the entire car wet so you can't tell which bits are actually clean.
Then, when the car dries, you end up with a lovely clean/dirty pattern wherever you've sprayed the water.At worst, the pressure is high enough that it can strip the laquer off your wheels, remove paint, peel stickers/badges and damage rubber seals, not to mention the damage you can do if the jet finds its way into the engine electrics etc.
If you hold the lance far enough away to be safe then it won't clean properly.
If you get close enough to clean the muck off you risk damaging paint or laquer.My old man used to run a Kew franchise
(for the NW of the UK) and we always had jetwashers around the house as a kid but I've ALWAYS found them more trouble than they're worth, in use.