WFP can be very good, it's problem is consistency, no matter how good you are, you will still make many more mistakes than you would with trad.
That's a fact.
It is far easier to do a bad job with WFP, with trad you are up close and personal, when you finish a window, that's it, it's done, complete.
With WFP you walk away with it dripping wet and it may or may not dry out perfectly.
I have been WFP for about 7 years now and would not dream of going back to 100% trad.
I do a first rate job and absolutely swear by WFP, overall it does a better and more thorough job than trad alone, but there are now more windows with errors on them than there were prior to me going over to WFP.
The drying process is subject to the weather, if it is windy and there are trees and shrubbery around then the potential for windows drying out with spots on them is higher, ditto if you are close to a busy road and the roads are wet and dirty.
The more experienced you are the better you get, you identify windows that will cause problems, you learn how best to deal with oxidised frames, but you will still get caught out on a regular basis.
And of course there are some windows that just do not come up well when cleaned with WFP and sometimes that can be a window that is right next to an identical one that comes up perfect!!
The plus points for WFP for out way the negative points, unfortunately there are some muppets out there that seem to think that just to have WFP makes them a brilliant window cleaner, swish a brush over a window = job done.
The Lloydstsb branch in my town is done WFP, they've just had a full refit, lovely, freshly painted window frames that are perfect for WFP but the quality of the job done is absolutely appalling!
No way on this Earth would I ever do a job that bad, but then, I'm pretty fussy, and if you are too then by and large you'll do a first rate job...
All comes down to the operator in then end....
Ian