Canvassing companies do exactly what their name implies. They go out and do your doorknocking for you. What they provide you with is a list of people who have said they are interested in having their windows cleaned.
The fact that some of these people will turn out to be time wasters or even complete nutcases is just a fact of life.
The trouble comes when a list of names starts getting offered as something its not (ie a working round ) and sold to someone too inexperienced to know the difference. This often has the element of scam to it.
But it is not all one sided. Many of the buyers are not capable of working the round and it falls into decline. Weeks turn into months and then it changes hands again with more problems.
My view is if you have the personal qualities to run a round, turn out on cold mornings, deal with the muck and truck, keep your customers happy etc you are going to want to do your own canvassing.