After numerous testing, I've determined that player hit effect buffs on equipment isn't working the way it should. spiky collar and other custom made gear don't apply thier effects when the player is hit.
Wish I'd seen this thread earlier. Would have saved me from letting an entire zoo pound on me to test an item. Anyhow, I can now also confirm that the <playerHitEffectBuff percentage="50" name="Weak Spiky Hit" /> line on the spiked collar isn't working, and indeed neither are my attempts to use playerHitEffectBuff on another amulet in my mod. Any word on whether or not this will be fixed in the next patch?