Even ignoring Rend and such, a look at some 10N Ultraxion parses shows that Unholy can expect about 55 chances at a proc every minute, and even when avoiding Mastery (as a non-Gurth UH will be doing) they get about 30% extra damage from their Mastery, so that's equivalent to about 70 chances/min.
Arms looks to have about 78 chances/min, counting their Mastery.
Fury, wielding one, has about 65 chances/min.
Ret, without Seal of Truth has ~40/min. With it, ~95/min.
So, 70 chances/min, or equivalent seems to be about the median. This gives the following variations:
SPEC. Chance/Min % of median
UH 55 71%
UH inc. Mast. 70 100%
Arms 78 111%
Fury 65 93%
Ret 40 57%
Ret inc. SoT 95 136%
Ret with SoT was less variant from the median than the new, nerfed, Ret, and by a considerable margin.
That's called 'overnerfing', folks, especially when UH will put a higher priority on Mastery, increasing their DPS from the sword (see EJ's UH thread), and Arms seems to get more procs than I assumed.