I don't enjoy playing Chopin with two hands because his music doesn't lie very well under my fingers, and certainly not as well as Liszt.
I don't know if I'd go so far to say that Chopin is "unpianistic" -- I have rather small hands. For people who have "lion's paw" hands, it might lie under their fingers rather well.
Brahms was one of the greatest of the 19th century pianists, but he wrote his music away from the piano. It's for that reason that people call his music "unpianistic" -- but it's not really unpianistic, per se -- I think of it more as "expanded pianism" and if you're going to play the 20th century composers, you really need to get Brahms with his "expanded pianism" under your belt. Being able to play Brahms securely and confidently makes composers such as Stravinsky, Hindemith, Shostakovich and Messiaen much easier to play.
I think Schoenberg's Suite, op. 25 is one of the most unpianistic things ever composed -- and just downright ugly music! Webern's Variations, op. 27 are not pianistic, but once one has the musical lines in one's ear you can redistribute the notes between the hands and that makes it much more pianistic.
For music which is thick, doesn't lie well under the fingers, and is just awkward to play, nobody tops Ralph Vaughn-Williams.