Unrest management is simply impossible with factions like the Britons or Scythians later on
Not really. Its admittedly much tougher for barbarian factions.....but doable. In my last few campaigns with Germania, Scythia, and Brittania, I had most of my cities in the yellow, at least, and some in the green.......and most without governors. Achieving ZPG is the key. Once you get that, you don't have to worry about revolts.