late answer but i think rustica has such experience playing that he's good at most compositions intuitively! i think though, there is one thing that i can think of and it's gonna be some classic me making things about west ballad given any excuse

but in west ballad ch10 rustica talks about an old composition he made that was only playable by a wizard because you would need at least 3 arms to play, but a human tried to play it and broke his fingers in his attempt to do so so rustica burned it.

so my guess is that if there was ever another song like that that required more than 2 hands to play it, it wouldnt be immediate perfection on the first try because its something he's only ever done once. i hope that answer makes sense!