From above - (how does one put it in italics??)
Certainly one instrument considered difficult to play is the bandoneon
Bing an Anglo concertina player I found it not too bad at all.
I think the hardest is Jews Harp - easy to get a simple 'twang ' devilish difficult to get a good melody - listen to any of the Wright family - (John, Micheal, David and ?Lucy?) to hear how wonderful it can sound in expert hands - (or mouths!)