It depends on your enemy.
I play BoK A LOT so I can give you my opinion.
Bloodthirsters are very powierful if you face a CC oriented army like SCE, BCR, nurgle, StD and so on but id you face Tzeentch, KO - they just die before they can attack. Huge point sink in those cases.
Most of the times getting more than one isn't worth the points.
Council of blood has potential but need to be executed properly. Combinig CoB with GOre pilgirms f.e. is great. You have two drops with buffed BT (to hit or save - either one is great) and You will start the game in most cases.