I dont think blood hunting and clamming are so comparable. An enemy can see and stop blood hunting with attacks and remote spells. You need to protect each blood hunting province from remote attack. The amount of effort to make blood hunting unbalanced can be matched by others to thwart it, but using clams isn't exactly hard, or easy to stop. That, and I would rather have a player have 1000 slaves than 1000 pearls.
Fever fetishes require a lot of micromanagment, and massive amounts of fire gems arn't quite as devestating as pearls, or even slaves.
I am for the limiting of clams. (Or, if someone could make a mod that makes them work like fever fetishs, but produce pearls instead
)