"The overwhelming majority of broadside songs are unsingable"

I think this depends on the era, Jim. The kind of stuff found in the Pepys and Roxburgh collections of broadsides from the late 17th century is indeed unsingable, at least by today's standards. Excessively long and wordy, sometimes with lines that are very difficult to scan. 19th century broadsides, on the other hand, are much more concise, and often correspond very closely to texts collected in the field.