Apart from message groups i don't see why guilds/religions too can't have referendums.
Ducal referendums are most potential thou, it makes allot of ducal identification amongst knights if they feel they have something to say or directly influence it's politics.
I've been trying to achieve it since Madina was founded but it is very hard without referendums or other ways in which knights can influence or benefit the duchy directly.
It's a practical limit (how difficult to code and how much time to spend coding that vs other stuff) as opposed to any belief that they would be bad for the game.
