Wüllner's variations are characteristic of late Romantic pedagogical and concert writing, focusing on structural coherence and thematic development. The work is marked by a blend of classical form and lush, late-century harmonic shifts.
The primary challenge is maintaining a singing tone (cantabile) across the various textures and registers.
Comparable difficulty and character — good candidates for what to learn next.
16 Variationen über ein Originalthema (Op.19) is rated Henle level 5 of 9 — late intermediate repertoire, roughly ABRSM grades 6–7.
You should be comfortable at Henle level 5 — meaning you can already play level 4–5 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 16 Variationen über ein Originalthema (Op.19) lasts about 10 minutes.
16 Variationen über ein Originalthema (Op.19) 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 Intermezzo in A major, No. 2 by Johannes Brahms, Berceuse by Dora Pejačević, Au soir by Joachim Raff — see the full similar-pieces list on this page.