All slow effects (including the RNG epic proc) are mitigated more and more with newer expansions. You will see messages like "...was mostly slowed" or "...was partially slowed".
What is nice about this weapon's effect (and the Wavecrasher's) is they add a 20 DoT component. The benefit here is that you can SEE if the slow is on a mob and often which one, unless you have multiple of the same mob name. Very useful for those groups with out the "Inspect Buffs" Leadership AA benefit.