@rotorgun
I´ve read somewhere that some settlements (Jerusalem, Tarsus and some more) have a built-in unrest factor. Besides the usual methods (garrisons, temples, games & races, ....) there is nothing you can do about it. These towns are just harder to control.

@breeze
I´m not so pessimistic about the Ceres temple in the later game. Bacchus is not an alternative to me because it turns governors into alcoholics. Jupiter has nearly as high public order bonusses as Bacchus has. But if a settlement is unable to reach 24000 people without the Ceres temple, you wouldn´t need a very high public order bonus from Jupiter. Therefore I build Ceres in all settlements with a natural farming bonus lower than 3% (can be found out in the settlement´s detail screen) and leave them all the time. It gets you several thousand tax payers more. The Arcani, that Jupiter delivers, are nice but so good that I would want to build them in every settlement.