Just one of the problems, and I'm sure someone or everyone will come up with more.
With a long run of duct like in the picture (a neglected one, which I suspect that one, was) it takes way to much water to get it cleaned. And it can be very difficult to handle a wand long enough to reach it all. If you think crawling through a greasy duct is bad wait till you have sprayed 40 gallons of water and chemical in it to find out it is not working and now you have to climb in. NOT FUN! They always seem to leak at one or all the welded joints so if it will take a lot of water that is also a concern.