Depends on the person too. Seems some people take naturally to some intruments and not others.
Just to throw one in. How about the Chapman Stick (like Tony Levin plays somtimes). It's a relatively new instrument so maybe it's hard because there may be a lot of uncharted territory with it. Never tried one though.
From the website: http://www.stick.com/method/
"The Stick method enables live execution of complete musical concepts, from the bass on through chords and melody, to the effects, ambients and sweeteners, greatly enlarging the musical scope.
Guitarists can now play lead lines backed by their own rhythm and counterpoint from all registers.
Bassists can support a group with driving low frequencies while filling in the spaces with chordal upbeats and melodic patterns.
Keyboardists will discover a hundred subtle elements of expression, fingers directly engaging the vibrating strings, and will excel at two-handed independence.
Drummers can apply familiar rhythmic techniques and reflexes to the world of harmony, simply by measuring distances between the hands and between the fingers on each hand. "