I received Hunger of Crota on Saturday night. I don't have Gjallahorn, but I do have Dragon's Breath (and Truth, though it is void and really doesn't apply the same way to your comparisons). I haven't really tested Hunger of Crota in action, since I only just received it. Seeing as it has tracking and does not take up an exotic slot, wouldn't it be preferable and/or of equal use as Dragon's Breath in many of the same contexts as you describe? I ask this question sincerely since, as I said, I have not had much chance to use it, and it is not yet upgraded.