On the melodeon or bodhran, it's relatively easy to reach a passably mediocre standard of performance (playing them well is another matter), hence the abundance of mediocre players. The same may be true of hammer dulcimers - don't know about banjos or piano accordions.

Highland bagpipes have their own peculiar scale, which sounds out-of-tune by standards of conventional Western music. Most of us, therefore, can't tell if they're being played well or not.