I don't give a damn if Merrill isn't more powerful, I just don't want to be forced to do things I wasn't intending to do. Do not seek to control my character's actions.
But that's exactly what demons are trying to do.
It's ironic.
The biggest proponents of blood-magic want it in the game and what it "done right" (UNLIMITED POWER!!!), but don't want the negative consequences.
They want those to be nuisances at worst.
Maybe if a demon tries to posses you, we can have a tug-of-war minigame that even a 2-yer old can with with his eyes closed?
If you'd want to TRULY model blood magic, you'd have random attacks from a demon who's sucess would depend on your current mental state (which ironicly isn't modeled at all) and it's power (which would be random).
You got drunk? Penalty to resist possesion.
Something shocking/tragic happened? Penalty.
Didn't get enough sleep? Penalty.
Area with thin veil? Penalty.
High willpower? buff.
Etc..
That would mean that a bad roll = game over. As it should. And you'd have to re-load.
But we can't have that, people would scream bloody murder
Really, a random chance for your character to be possessed or to draw HP from your allies seems cheap and artificial. I think the potential to really maximize its power should be made available, but it should be up to the player how far they really want to go.
Other way around.
It is cheap not to have it.
Granting the PC (and his party) total immunity to the danger the rest of the world faces is cheap, and presetns a skewed view at the danger of magic.