Possibly what your friend means is that folk music, especially trad stuff, is short on modulation - that is, when a tune in one key goes into another (often from a major key into a minor) and then returns to the original key within one musical phrase. This is certainly rarer in folk than in jazz, but it does happen, both in trad song melodies and dance tunes. This may make for more harmonic variety but, like most variations, can be overdone. For my taste, much jazz and modern classical does overdo it. Sometimes simpler is best.

Andrew