I'm a big fan of the theories I learned in my political science class once upon a time: democracy may increase overall happiness for a time, but the increased red tape means they are slower to move, even if they are less likely to face rebellion. Furthermore, because of the extreme nature of people's preferences (especially in TW), I can't foresee a way in which people would choose to continue following leadership when a mistake or unpopular decision was made. Unlike the real world, there is no insane amount of issues one could point to as more important, the people in a tribe find it a lot easier to organize and have their voices heard, and there's no real barrier to simply leaving. Lastly, there is no valid checks and balance; it relies only on the ability of players to actually follow the rules proposed to create the democracy, and if a player chooses not to listen to (as an example) the council and declares war as a duke, there is no real easy way to "impeach" them, you'd have to abandon ship.