The father in law tried this in conjunction with his omni vac on a bungalow a few years back, yes it removed the moss to a degree you could use algoclear to finish the job, downside it took them a day and a half (one side only) even with various scrapers, brushes etc..
Oh yes, if was really hard work as well!
Personally for a couple of old barns hire a cherry picker and blast them
Darran