The points made in this comment are fair, but really this was not what we were trying to talk about in the article. More or less by definition, a virtuoso performer on any musical instrument is doing things at the limit of human abilities. Most players can't match the experts, so in that sense every instrument is "hard to play". We were trying to make a different point relating to the underlying physics. A complete beginner can pluck an open guitar string or bow an open violin string. The guitar string can be more or less guaranteed to make a musical note with the expected pitch, but the violin string may produce a horrible squawk: the physics of bowing allows a wider range of possible responses than a plucked string. The player has to learn to control these, in a way that has no direct analogue with controlling a single note on a plucked string.