Instruments are forbidden. Not because they're 'bad' — because they're dangerous.
Three Reasons
Pride. An instrument singles out one person. They 'perform' — others listen. That's Hochmut (pride). In Amish worship, all are equal. All sing. No one performs.
Entertainment. Music with instruments is entertainment. Entertainment is worldly. Singing hymns is prayer. Difference: entertainment — for self, prayer — for God.
Connection to world. Instrument must be bought (economic connection). Must learn to play (educational connection). Can perform (public connection). Each step — closer to the world.
Exception
At home, privately, some communities tolerate harmonica. Quietly. For oneself. Not in public.