You are certainly right. I have seen it too. I would never use the Karcher on a roof, too much risk of water getting inside the building through tile gaps. Even worse that you could cut straight through the membrane if you get an angle wrong. And i don't particularly want to have my K5 up on a roof.
I will not use a pressure washer, I will something a lot softer and less fierce until she reads this... I will get the wife to do it...