In practical terms most commercial (except the smallest) or local government premises will require you to meet H&S regs. They will not want you up a ladder.
On domestics no one is likely to stop you and take issue over your method of working - many householders don't know the regs. and will trust you to know what you are doing and the older generation often do "not like" wfp.
But when you have an "incident" - a slip or damage property then the sleeping dog will awake and depending on the seriousness of the matter than you could be held criminally or financially liable if you have not chosen the reasonably safest way to work.
Most w/c's who have looked at this issue without preconceived ideas tend to realise that wfp is the safer method. But....
It could be concluded that trailing hoses - if not properly "signed" - could be a risk and that water freezing could be another.
So if pure safety was the motive then perhaps a trolley system for upstairs and blading for downstairs might be the safest way. But then you have more "back issues"! Lifting 25kg drums or trolleys and car batteries about needs care - as does handling a pole or carrying a ladder.
I have used ladders, a trolley and now a van mount.
Generally I think van mounted wfp with bright microbore hose, adequate signs, a light pole and being prepared to salt and/or blade doorways or near vulnerable pathways in icy conditions is the way to go. But speed is also a motivator for me!
I truly feel that ladders are now unnecessary as a general rule of health and safety and believe that wfp versus ladders lets me safely reach far more windows than it cannot.
Windows over a long flat roofed extension are sometimes the only difficulty I encounter and although I don't personally do them some justify a ladder for that alone.
Having said that I will use a six foot ladder to let me reach over gates that are locked or blade above a door set in a porch.
Hmmm, that post was longer than i meant it to be!