The problem I had with using a waterbroom, is that I was using renew from Ace cleaning (same as pitchwitch), and I would apply the chems, and let sit for 15-30 minutes depending on weather, and would then rinse. I always seemed to go over the same places over and over, and would still have to touch it up with bleach, because it was so much trouble to Xjet it using the PW.
As for the ground guy (my ground "guy" is my wife), She makes sure that everything is good and soaked before we start, then as I apply and rinse, she takes special care around dowspouts, and on concrete and decks. Plants can usually handle some drippage from chems, as long as you wash off with in about 15-20 minutes. Concrete and decks seem to discolor immediatly if you do not constantly flood them AS the chems are dripping. I learned the lesson the hard way, when I moved to the side of the house with the driveway, and didnt tell her. By the time I was done, the driveway was streaked like crazy. It was a million dollar house, and they weren't all that happy. Loved the roof though!!