My understanding is PVCU frames are designed with water channels in them with drainage holes and sometimes external covers depending on the design of the frame same as dg units in doors.
The dg unit should be fitted correctly so sits on spacers that does not restrict the water flow so any water/moisture getting past the gaskets so can drain away properly through the drainage holes. Obviously blocked drainage channels/holes will also not allow proper drainage too.
Any water/moisture trapped in the frames will find its way into a dg unit if it's not manufactured properly and/or starts to break down. This will ultimately lead to water/moisture getting between the panes of glass (blown sealed unit).
That's why I say using wfp window cleaning method can actually speed up the process on failing dg units due to the amount of water used hitting the glass, gaskets and frames.
Regards to water ingress into the building, drafts etc this is due to the frames not been installed and sealed correctly and/or the sealant starts to break down over time. Again using wfp method on frames of this nature can cause water ingress and damp on the internal window returns too.
Apart from that wfp methods is great but still it's not our problem if a customers windows mist up or they get damp inside