Superb bits of kit if used right.
Pros : flexibility - you can go awkward places more easily than a full van system
Very cheap to start up and if you like it, you still have poles, brushes etc to take to van system
You don't have to buy, tax, insure a van dedicated solely to window cleaning
Cons: not many really. Time spent filling. Loses me 1 minute every 20. No big deal.
Have to filter water at home so time spent in the evening.
Shurflo switch has gone on both mine. £48 ish to replace.
I know there's always the "who can earn the most" type threads on here, but with the right customers you can do very well with this type of set up.
If you want to try wfp, I wouldn't argue against a backpack. Remember you need to treat the water. Depending on your water type you may need RO system or just a DI vessel The info is all out there.