Nah, not worthless. Just not great for ores. Paired with Arcane Bellows the vis/flux impact is hardly noticeable. Paired with a Brain in a Jar it's a great source of on-demand XP. I use a Hungry Chest to catch the output.The infernal furnace is inferior in bonus to both the macerator and pulverizer (my testing with three bellows was that a stack of ore turns into a stack of ingots plus 84 nuggets, a bonus of 9 ingots where the macerator would give a bonus of 64 and the pulverizer 10% more than that.
It does smelt dusts, so you can use it on that, but provides no bonus. Note it also eats vis and creates flux.
In short: In a mixed mod game, the infernal furnace is worthless.
I find it's great for free smelting of sand into glass and cobble into smoothstone. Not sure how many stacks you can throw in there, but I've tossed as many as eight at a time, IIRC. And if you use it to cook meat you get meat nuggets.