The theramin is, without question, the most difficult instrument in the world to play.  Consider: since its invention in 1928 and commercial release in the early 1930s, only one musician - Clara Rockmore - has been able to produce any real music on it.  Her performances are spellbinding and, if one sometimes years for, say, a whole sonata or something more substantial than encore pieces, think of how difficult it is to play ANYTHING on this instrument and be glad that we at least have this much.