I think this question is one that cannot be answered, even though some modernist works are almost unplayable (ie Finnissy, Messiaen, etc). They have their own difficulties. The more difficult romantic concertos, such as the Henselt F minor, also have their own difficulties. Even "simpler" concertos, such as the Mozarts, have their own. Sometimes, when you are playing something "simpler", it is harder to mask your techincal slip-ups than when you play, lets say, Messiaen's Oiseaux Exotiques. The sheer volume of notes and lack of inherent tonality are enough to cover up slip-ups, whereas, in the Mozart, everything is left out in the open, per sé.