Acoustic music presents the most difficult challenge for live sound engineers. Very often, excellent acoustic artists can have their concert ruined by "rock-and-roll" sound engineers who simply aren't used to dealing with the intrinsic nature of acoustic music. Precise and attentive engineering is required to avoid feedback and obtain a good sound both on stage and in the concert space.
Folk festivals also provide an excellent training environment. Expect critical artists, attentive audiences and very fast change-overs. It is not unusual to engineer 15 acts per day, each with varied arrays of instruments and set-up, yet all requiring first class sound. And its not all "diddley-de" music either! Folk and world music now can include drum kits, keyboards/synths, DJ decks as well as fiddles and accordions!
Over the years I have mixed a wide variety of music style, and I can say categorically that acoustic/folk/world music is by far the most challenging. By developing an engineering skill-set in acoustic music, it is possible to translate those skills to other areas of live sound reproduction with ease.