Rzewski's piece is a deconstructionist commentary on the tradition of the piano cadenza. It utilizes complex, often aleatoric textures to challenge the performer's relationship with standard musical form.
The primary challenge is navigating the dense, avant-garde textures and the intense physical demands of the high-speed sections.
Comparable difficulty and character — good candidates for what to learn next.
Cadenza con o senza Beethoven is rated Henle level 8 of 9 — concert repertoire, roughly diploma level.
You should be comfortable at Henle level 8 — meaning you can already play level 7–8 pieces cleanly. Learning it one level early is possible as a stretch piece, but more than that usually leads to months of frustration.
A typical performance of Cadenza con o senza Beethoven lasts about 10 minutes.
Cadenza con o senza Beethoven is available as free public-domain sheet music on IMSLP — the score is linked directly from this page.
Pieces at a comparable difficulty and character include Ballade No. 5 (It Makes a Long Time Man Feel Bad) by Frederic Rzewski, Blah by Frederic Rzewski, Ballade No.6 by Frederic Rzewski — see the full similar-pieces list on this page.