"+20% duration of hostile effects on nearby enemies on scoring Kill"

Working from the same rule that Strand of Favor do, that Frostball enchantement can increase the duration of foes debuffs, keeping them paralized or stunned (for exemple) "for ever" .

For ciphers, it mean the duration of linking abilities like Psychovampiric shield , Body attunement, Borrowed instinct... will grow for him by the way ;

this is not only reliable on Big number' foes fights, because killing your squelettons trigger the effect in the area (maybe 4 meters around the target, maybe), so even a boss alone can be stay in respect .

Dual wielding two Frostballs to economise Brilliant

Nice. Does the killing blow have to be done with Frostfall itself or does any kill (DoTs, spells, Carnage etc.) count?

Just the frostball killing blow, and any abilities that use it to kill, so not carnage raw damages but yes for soul annihilition raw lash damages. Yesterday I was in the Hole tavern in Neketaka and the ogre I choose for my Borrowed instinct was at more than 300sec for that in less than 3 minutes..

I think rogue, monk and devoted are very convenable to use it.

Sure - that just means that you have to actively target allies (like skeletons) if you want to use them as fuel for Dispersed Suffering. It can't be done by AI and it can't happen "accidentially" via Carnage for example. Confused Clear Out and Cleaving Stance would work without specifically target allies though. Heart of Fury as well. Belt of Magran's Chosen would bring some weak enemies that can be "harvested" for more durations I guess. 

In encounters with enough weak enemies you don't need all that of course.

And I guess in encounters against bosses it's not too inconvenient to actively kill a few summons.