I think what you can gather from the above posts is that wfp can be made to work, but it needs preparation, both of yourself and your equipment. A properly set up system will not give you any problems. In fact, after 5 years I'm still on my first pump and controller, even though I now have a GrippaMax system/DIY install. Ask lots of questions and think, sometimes out of the box. Learn simple troubleshooting, step by step isolating the problem. It's not rocket science, but there is some knowledge involved. Carry some basic tools, and minor spares.
Same for the technique, it can work, and will work very well. It's all in the preparation and details.
Yes, you can do a good job with traditional tools, but the safety aspect is a thorn in the side. It's a very inefficient way of cleaning windows. The time you take to get up and down your ladder and moving it has got nothing to do with cleaning windows. With a pole you can practically keep cleaning windows, bar the occasional niggles of laying out the hose.
It will work, do the research, set it up properly, and train yourself and your customers. Hang in there.
I would slightly disagree with the comments on trolleys. A trolley setup will not get you the best start in wfp.
For one, you will always skimp on water, whilst this is exactly where you need the water the most, to get a proper flow, to be fast, and to clean efficiently.
More over, it's a hateful thing to keep lifting a dead weight out of the van, and rolling it around you, barrels and all. Refill every 10 minutes or less.
Van mounted system every time, it can be done on the cheap. A good trad window cleaner should earn enough money to save up for a basic van and system. Dump the trolley. If you have an awkward job that needs a trolley and backpack, you are better off without the job.