[The harmonica] excessively stimulates the nerves, plunges the player into a nagging depression and hence into a dark and melancholy mood, that is an apt method for slow self-annihilation. ...
If you are suffering from any nervous disorder you should not play it.
If you are not yet ill you should not play it excessively.
If you are feeling melancholy you should not play it or else play uplifting pieces.