The relation system is another issue, the fact that you gain around 30 influence daily if you own about two fiefs, shows that it is out of proportion compared to all the other methods of gaining influence combat, buildings, other policies like Senate or Castle Charters, etc.
Having two cities is like winning 2-3 major battles daily without doing anything influence wise with this policy.
Not to mention the game breaking factor for vassal playthroughs as the king gets do to do whatever he wants, and your faction becomes way to powerfull due to high garrisons.