Hi, check the professional detailers sites, some use them some dont, they will say that its the person using them that the problem, stand and watch the high street washers and you will see what I mean, dont copy anything they do.
They are too close and washing at the wrong angle i.e. jetting water straight on to the panels, you should always spray at an angle that hits the crud at a shallow angle hence washing it away. Spraying straight on will force the particles on to the surface. I watched them washing a nice black SLK around the wheels at a distance of less than a foot and they blew off the centre wheel cap clear of, they are permanently attached to the wheel.
You do not need high pressure and the karcher will do the job and you can get snow foam attachments for them, if you have the money then I agree the nilfisk is the better washer. You do need a washer if you want to snow foam.
Remember though its the user not the machine, also never final rinse with one always sheet your water.