i'd never heard that said about either, will make for interesting reading further research on my end later, the only references to insanely difficult works for piano are the ones i have come across regarding the Strauss Burleske for piano and orchestra,
usually reads something like this (lifted from a CD review featuring the rarely recorded work)
"...Marked as extraneously difficult and even unplayable since its composition in 1886, Richard Strauss' Burleske for Piano and Orchestra rarely finds its way onto CD programs and even less frequently in live concert halls. Much like the extreme demands Strauss places on orchestral musicians, the technical feats called for in Burleske are truly Herculean; 10-note chords, huge leaps, enormous reaches, and rapid filigree passages fill the 20-plus minutes of music. ..."
i heard this live years ago it blew me away, one of those works that anyone can make look hard, few pull it off with finese convincingly