Uhm. Considering you can hunt with 8 scouts in a province with 0 tax and have virtually no unrest problems. It's not that unclear. On the other hand anything over 4 1 Blood with SDR's will ramp up the unrest beyond what is feasible to use taxes to control and you'll have to use patrollers.

Scouts are both more cost effective, time effective, safer and universally usable than blood magic blood hunters because of this.

There are only a few nations with the ability to hunt 'better' with less loss while at the same time not sacrificing something.

Other than that and needing a specific amount of Blood Slaves per turn for your progress, there is no reason not to use scouts except for the micromanagment aspect which could be nulled with a simple "Pool to Commander" command.