JeffB-
Oddly enough, it's quite possible to compare apples to oranges. Oranges are generally rounder; apples are less juicy. Oranges are a better source of vitamin C. etc.

Similarly, I see no reason why one can't compare vastly different ballads. There are some fundamental differences between Tamlin and Leader of the Pack; there are also fundamental differences between Tamlin and Our Goodman.

What you're saying is basically what I'm saying--except that I find "ballad" a more convenient term than "narrative song". I'm all in favor of sub-classes.