IMO Bone Spear/Spirit have too many synergies.
Teeth, Spear, and Spirit should all synergies with each other and that’s it. Modify the base damage and synergy % accordingly.
On to your main point. Holy Bolt and Bone Spear work exactly the same so the comparison is a good one for things besides act bosses (see below). Spear should deal less damage, but not a massive amount less.
Spirit is what Bone Necros use for bosses anyways, so for killing act bosses, compare holy bolt to Bone spirit damage.
Anyways, I think they are missing an opportunity with their approach to FoH/Holy Bolt and… Sanctuary.
Check it out: