I was out hunting trophies on my dragon and a largish spawn of Marble Boulder Golems spawned around me; there were probably a good 7 or 8 of them, so I went and rounded them up and hit primal cast, figuring I'd BOFB them and then let the primal cast trigger on my next spell, Drulkar's wrath.
To my surprise, I didn't need my triple Drulkar's Wrath to finish them off, as the Breath of Flame Burst used up my primal cast and wiped out every last Boulder Golem. That was an average of 1100 damage X 3 per golem; I don't think a single one was alive to take damage from the 3rd BOFB. That BY FAR exceeds any damage a high powered mage can do, using it's highest damage spell (in conjunction with burn armor, fusion burn, etc.), also multicast.
It was my understanding that only spells could be multicast, Burning archer can't be multicast, phoenix rising used to be multicastable, but that got changed. I really don't understand why BOFB is primal castable. Even the description of primal cast says ranged only spells only.