Dave,
I've been WFP for about three years now. At first I worried about getting complaints, and like you I spent ages going over and over the work to try to avoid them. I still got a few.
I have a row of houses that were underquoted badly when I took them on. Out of pride I kept them and made them pay by working hard (trad). When I changed to WFP I put the prices up by 25% but they were still too low, so all I do is slosh over them with the brush and give a quick rinse.
I've never had a single complaint from them and they always look brilliant.
The answer is that some jobs will never look good done by WFP, and others will come up perfect every time with a "lick and a promise"
Try to find out which ones they are and do them as I do my "cheapos", any dodgy ones either get rid or be very careful.
Cheers,
Ian