"Unrest accumulates from unrest. So if you are unable to completely kill the unrest of a turn by tax or patrol, it will breed more unrest. So if you have a set tax rate that will not clear the unrest you currently have, produced by your current unrest, and will squelch the unrest caused by next turns blood hunts, you will accumulate unrest.
An unsuccessful blood hunt gives you 1 point of Unrest. So even if 5 scouts fail, you will only get 5 points of unrest, which will be cleaned by 90% taxes."
Are you sure about this? I was not aware that unrest produced more unrest, I thought that it was the tax rate (sites and other events nonwithstanding) that controled unrest. I'll have to pay more attention to this, but I've had provinces with ~10 unrest where I just left it at 100% tax and saw no movement in either direction...
Further I'm not sure that a failed blood hunt gives only 1 unrest, I'll have to check it more closely, but I seem to recall a ~4k province where I had 2 scouts blood hunting where the unrest would jump anywhere from 0-10+ on a given turn (even with both scouts unsuccesful), and this with the 90% tax. Granted I hadn't done a complete search to see if there may have been some site present...
I will try scouts out some more though, perhaps my test games where I employed them just were unlucky, or perhaps you've just been luckier
There is flexibility for either system, and I do tend to augment my research abilities in other ways if I do plan on using my national mages to hunt early, then again to get to Blood3 and Const4 isn't all that difficult usually for early game