I'd agree with Neovo that you're much more inclined to make the extra effort to play an instrument you've fallen in love with, than one with which your frustration mounts and eventually gets the better of the intial attraction.

This is another aspect of the debate as to why people play the instruments they do: what one person would find the hardest instrument in the world to master, others take to like ducks to water.

I have spent hours and hours on melodeons down the years, and still cannot play a single tune at the slowest of snail's pace with sheet music, tablature, and intense concentration - yet I've taken to English concertina reasonably quickly in the last few years, and can play that, and several wind and stringed instruments, to a reasonable public performance level.

There are viruoso players of all the instruments that have been mentioned so far, and I'd wager that each one of those musicians would find one instrument in particular that they just could not get to grips with at all.

Empirically I'd agree that uilleann pipes are probably pretty close to the top of the pile, purely because of the sheer number of different physical activities you have to perform just to get a note out - but even then I'd suspect that some pipers have sweated blood to get to the standard they have reached, whilst others were just 'naturals'.