The furnace or its power is typically the most effective effect to add to any build. It is incredibility dull when compared to something like the corrupted ash bringer. Unfortunately to maximize your build potential, you add furnace.
Let's petition blizzard to make the more imaginative and interesting weapon powers as effective as the furnace for builds.
I don't understand why Furnace has the elite dmg as legendary affix while Sunkeeper just has it normally :/
because sun keeper has always rolled elite damage, it was one of only a few items in the game back in classic that rolled it as a stat, so it has been carried forward into RoS as was, the furnace is a "new" legendary, and because the stat value of elite damage on it is so high, it has to be as a power effect rather than a PRIMARY stat roll, which is what makes sun keeper generally a bad weapon due to the stat restrictions, it is much more difficult to get a well rolled ancient sun keeper than it is to get a well rolled ancient furnace all things being equal.
I think the real point is that the Furnace's legendary affix is not creative; its just an absurdly high roll of a rare and powerful affix (which usually rolls at 15% at most) as a SECONDARY effect. Its brutal and utterly simplistic power is almost unmatchable.
I love Scourge, but Furnace 50% damage to elites is far more powerful, because it affects all of your damage, so while scourge is really great against large groups, furnace saves you immense time against elite packs and guardian, which takes up a lot of time. So I throw Furnace in cube, but I actually wield Corrupted Ashbringer. It is super powerful.