The proc has a -50 MR resist mod on a 180 DD, which is perfectly inline with procs and effects of that level. For reference:
Lash of Chaos, found in Sep and CoD:
150 DD, -100 magic resist
Dreadflame, found in CoD
150 DD, -75 magic resist
If anything, Lash of Chaos needs the resist mod toned down. Rogues are trading resistability for a little more damage, which seems fair.