some really filthy roofs won't come up with wfp they need to be done by hand.wfp is good for rgular cleans but to restore you need a ladder.
On stubborn cleans I use TFR and detergent tablets in a mixer bowl, very hot WFP and very hot pressure washer and haven't come across one I haven't been able to clean yet, although I may one day. I'm looking into steam cleaning alternatives for the plastic to make life easier.
If you was doing a normal clean, not a heavy duty, filthy, black or green clean would you WFP it? If it was a really bad clean would you WFP the easy bit and hand clean the worst off?