These variations are a significant example of Berg's pre-dodecaphonic style, reflecting a bridge between Brahmsian structures and the Second Viennese School. The harmonic language is highly chromatic and atmospheric.
The performer must focus on managing the wide dynamic range and complex polyphonic textures.
Comparable difficulty and character — good candidates for what to learn next.
12 Variationen über ein eigenes Thema is rated Henle level 7 of 9 — late advanced repertoire, roughly ABRSM grade 8 and above. In the RCM system it corresponds to about level 10.
You should be comfortable at Henle level 7 — meaning you can already play level 6–7 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 12 Variationen über ein eigenes Thema lasts about 6 minutes.
12 Variationen über ein eigenes Thema 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 12 Variationen uber en eigenes thema by Alban Berg, 12 Variationen uber en eigenes thema (1908) by Alban Berg, Ballade en mode phrygien, JA 9 by Jehan Alain — see the full similar-pieces list on this page.