First cleans hard work, you really have to work at it, and when you start it is such a steep learning curve
The overall finish is at least as good as you will get with traditional methods, you don't need to over-promote the health and safety issues, you have to be gentle with the poor old customers, they have to learn that the job they are getting done is a professional one.
Of course spotting can occur, but you get problems with trad methods too, runs will occur, so will smears, the frames will never come up as well as with WFP and if detailing isn't done properly (when needed) that will show too.
With WFP you can't be 100% sure you have done a top job because you are leaving the glass wet, but as you gain experience you begin to know when you have done a good job.
It won't totally replace trad methods, some windows, notably those with oxidized frames, do not come up well with WFP, but most windows are UPVC now, and not that many houses are in that poor a condition that the frames have oxidized.
If you do a sh*te job then you haven't done it correctly, that isn't a fault of the system, that is an operator fault!
I would recommend WFP to any serious window cleaner, domestic or commercial. You may well lose one or two customers, but you will gain more than you lose, and your life will be a lot easier to boot
And once you have been using WFP for a couple of months you will really, and I mean REALLY! Realise how dangerous working off ladders really is.
The health and safety issues are almost incidental, WFP is quite simply a better way of cleaning windows.
I would never, ever, go back to full time traditional methods, I don't think many other WFP'ers would either
Ian