I think mercenaries should be reworked to work only for rich factions, because currently factions can use them as a "debt payer" for their factions debts, they should make it, like it is in Knights of Honor 2: Sovereign, who a Kingdom needs to pay a lot, to fight their wars, but after peace, they instantly leave the service.
From my experience mercenaries do leave pretty fast if the faction they work for is at peace. They will even leave a faction mid-war to work for one that is weaker if it is at war, even if that war is against the first faction.
Imo, I like them working for weak factions as it helps to balance them and gives them a shot, I just think they should be excluded when the game goes to take away money from all of the clans in a kingdom.
My biggest issue with them currently is this: I am at war with Vlandia. Vlandia is around 2x-3x my size, more powerful, more clans. Ideally, mercenary clans should be offering to join me, both because the game logic dictates they should assist smaller factions in war, and because I am smaller and there are more enemies, there's a better chance of them earning influence and thus payment. However, because I am not an option to them, they are actually joining Vlandia, tipping the scales way out of balance.
This is what I meant when I said:
this is a pretty major handicap to a pretty significant "core mechanic" in the game, which is running your own kingdom.
Its like an artificial handicap being placed against the player and makes it unnecessarily difficult and tedious.