I'm not sure on this one, I have done a fair amount over the years using a wfp but watched one of the
neighbours getting theirs cleaned a few months back with a power washer and it did seem to look quicker.
If I'm being honest it did seem to look handier as well.
As for the quality of the clean, it was every bit as good as a wfp clean.
Interesting that it looked quicker - given the speed that WFP can clean gutters, I'd assumed that it wouldn't be faster.
One thing I thought about overnight was that if you were blasting water up into the gap between the fascia and gutter, water would be getting under the tiles at the edge of the roof. Possibly not on newer houses but certainly on a 1920's place like mine. In the loft, I can see light round the sides. I'm not sure how much might come in but some would, I'm sure.
One thing I wouldn't miss is doing the brackets with WFP. One, they are fiddly and, two, one day one of them is going to come off.
Vin