WFP is a fantastic tool, and it can do a top rate job, no question on that....
However; It isn't as consistant as trad, when you walk away from a window you have cleaned trad you know exactly the kind of job you have done (mostly)
There is always a greater risk of getting it wrong with WFP, once you are a skilled and experienced user you will reduce that risk but there will always be the odd window or account you can't get right.
I currently use the lightest pole set up on the market (superlite + superlite brush) fabulous in use, but the downside is that it is fragile.
There is always a balancing charge!
Trad will leave you far more tired at the end of the day, it is way more dangerous to work off a ladder, ladders can damage sills amd mark walls, scrims will leave smears, squeegees can leave lines and kicks and so on and so forth.
So although you can acheive far more consistant results working trad, it isn't without its downsides.
Sometimes the problem with spots may be windy weather and nearby shrubbery or trees, it may be the rubber seals on the windows, it might be oxidised frames, and sometimes, no matter what you do, particular windows will just never be right
And equally often, the problem is the customer!
I personally refuse point blank to use my ladder other than for access, problems with downstairs windows? Then I'll use WFP on them, and then nip back over them with a squeegee.
This isn't as bad as it sounds, you don't have to detail or worry too much about technique, after all, there isn't much on the glass to leave marks because you have already cleaned them with pure water.
Upstairs the customers is told live with it or get another window cleaner (but not that bluntly!).
Ian