I have two customers who both had varnished oak bi fold doors fitted within a short period of time, I have never though it a good idea to clean these with wfp as i thought it would deteriorate the wood.
As an experiment I cleaned the bi fold doors of one customer trad and the other wfp. The doors cleaned trad are looking good still with minimal signs of aging, however on the doors cleaned with wfp the wood had started becoming discoloured, the customer even noticed and asked if I would clean the oak bifolds by hand.
Whats interesting is the doors I have cleaned trad all along are open to the elements where the ones I cleaned wfp are protected from the elements by the overhang of a roof. I also think that aslong as wooden windows are well maintained they are ok with wfp but wfp seems to accelerate the deterioration when they are getting ready to be painted.