Much of what you call "folk music" that is performed in Catholic churches these days is not really folk music in its strictest definition of the term. Instead, it's music that makes young people cringe because it sounds "so last year," which is the kiss of death. Real folk music is hard to come by and mostly not appropriate for the liturgy. I'm thinking about early American tunes, Cajun folk music, that sort of thing. Also, great music is not always difficult to perform, although that is sometimes the case. Mozart's famous "Ave verum," for instance is a great piece of music written about the same time he wrote his famous "Requiem," but it's not at all difficult to perform. The "Missa Syllabica" by the contemporary Estonian composer Arvo Pärt, for instance, is not very difficult either. Sometimes the best art is the simplest.