According to the Arelith Wiki, Shadow Mages get +1 caster level per shadowdancer level. However, this level is not being used to resist dispels. Consequently, a 20 wizard/10 shadowdancer is completely dispellable as if they only had 20 caster levels.
Steps to reproduce the Bug:
Create a 20 wizard/10 shadowdancer on the PGCC with greater or epic abjuration focus, cast any number of buffs, then dispel yourself with greater dispel.
1d20 + dispel's caster level(max 26) vs. 12 + creator's effective level (in the screenshot, I also took arcane defense abjuration, which should have added an additional +2, or 14 + 30 = 44).
Expected Result:
20% or less of buffs will be removed by the initial cast.
Actual Result:
90-100% of buffs are removed by a single cast of greater dispel.
Screenshot(s)
