Your domestic electric pressure washer will produce about 2000psi, the commercial car wash ones are at least twice that, compare with 45 to 80psi from a garden hose. if you are gonna have the pressure nozzle on wide spray and hold nozzle away from the car the extra pressure does nothing as the pressure of water hitting the car will have dissipated anyway. There are photos somewhere on this site showing radiator martrixes that have pretty much all the fins missing, number one suspect was high pressure washer used to clean them.
Most people advise not using a pressure washer on mountain bikes and BMX motorbikes because high pressure can force its way into chain links and past bearing seals etc causing many problems.
I have seen what even domestic pressure spray can do to concrete slabs etc, and the funny thing is that once you have cleaned moss and lichen off once it comes back much quicker after because you have basically destroyed the smooth concrete surface and roughened it up which gives plant life much more chance of taking hold.
If you love your car leave the Karcher in the garage.