Interesting. The way you describe it, it seems like it "saves up" fetish spawns until you actually use mana, and then "detonates" all the saved up spawns.
Tbh it's not really that bad, as I'm still using Haunt/Piranhado as spenders. Was considering a 3/3 HT/Zuni support doc. 23 fetishes (8 from FA + 15 from BoT/FS) for durability, together with the 2/4pc HT bonuses (increased damage done, reduced dmg taken). Zombie Dogs with Life Link and Uhkapian Serpent, Locust Swarm-Cloud of Insects and Bad Medicine for damage reduction, Haunt+BBV+SMK (23 fetishes, remember) for damage amplification.
If the Wormwood were to give me fetishes passively... I'll still test it out tonight though.