The problem here is that coalitions are a bad idea on a fundamental level, regardless of implementation.
Okay, that I disagree with. I personally just feel it should be limited to the utmost extreme cases, like e.g. the Mongols or Aztecs overrunning the entire known world. I can see Muslim Iberia and Christian France fight the Azetecs together, but not the ERE emperor getting crushed by the Pope and Caliph alright. However, that would then be (arguably massive) tuing imo, and not render the entire idea bad.
I have to admit though, that I mostly play with the CK2+ mod on, which has the entire vanilla decadence thing completely disabled because it didn't work properly. So I am not defending coalitions blindly, I am just saying "let's first see if they can be brought to order".
But that is our personal opinion, and while I don't want to imply that everyone here is raging, I am rather suggesting to put those things in a seperate thread, which I wouldn't call "United players against Coalitions", but rather "Feedback: Why Coalitions are broken".