Why do Merchant Republics have no estates? Seems pretty unfun to play as one now.
The dev team ran out of time to implement estates for merchant republics.
Sadly it creates a huge imbalance since the burgers estates give 20% trade efficiency and extra trade power. So merchant republics get 20% less efficiency than everyone else. They lose the burghers province trade modifier as well. So merchant republics now make significantly less that non other government forms in trade.
It's fairly game breaking and they really need to hot fix it.
They don't even have to implement estates for merchant republics either. They can just change the modifiers for the existing guild mechanic.
The bonuses are kinda crappy because they are outdated for the guilds anyway.
Edit:
I looked the existing bonuses.
Just give merchant republics a 10% trade efficiency modifier for governmen type.
Make the faction bonus more substantial. Like 10% efficiency for traders. Up the morale for aristocrat. Guilds get -10% development. Etc..
Maybe even more bonuses. You can get fairly good bonuses here because it costs monarch points to throw the influence around.
If they got time to program I would suggest a drawback of 25% autonomy in provinces that don't have an important centre of trade, estuary, trading post.