It was VERY useful in GC1 and GC2 but I've noticed how easy it is for the AI to exploit it in an unrealistic and gamey manner for GC3. To the point of them essentially removing it from the game as a thing that exists.
My first game I was still figuring things out and didn't get voted into power. So I had to suffer through some really weird AI proposals, some were alright... others meh. But nothing worth abandoning the UP over. Then later in the game they finally vote me in and no matter what I decide to do everyone abandoned the UP during the vote. Canceling it out. EVEN MY OWN ALLY when I picked a topic that was supposed to HELP THEM and not me! I wanted planets to become immune to culture flipping as I was about to steal one of their planets that we took over during out last war. Every single vote was someone leaving the UP for the rest of the game.
I look into the mechanic and see that well apparently there's some countdown timer where the AI will abandon the UP if they get X amount of things they don't like. I toss it up as just late game issues. So after I beat that game and start my second game, I finally get voted to lead the UP immediately. The first vote is fine. I make it so that anyone who leaves the UP is instantly declared war by the entire UP, it gets voted in.
Surely this will make it easier to not run into the same BS that my first game had right? Nope.
Second proposal gets thrown in. A race that has no more homeworld is looking for a new planet to call home, and I decide to vote yes for that. My vote is the majority. This doesn't really harm any AI... and yet the Snathi's Revenge decides that this is the vote worth having the entire galaxy declare war on them. They leave the UP, get declared on, and their neighbor the Terran Resistance obliterates them. How is that smart at all? What did that accomplish them? All it did was ruin any sort of fun I could have with the UP as a player. And I'm not even trying to game it myself... I'm picking stuff that's as neutral as possible. Literally trying to roleplay and do what's best for the galaxy.
So that was the 2nd vote, where'd the supposed countdown go? And if there is a countdown then how come the AI can chain leave the UP one right after the other? There's no cooldown it seems with this thing. It seems like the AI calculations to leave the UP are very poorly designed. They clearly aren't factoring in the fact that they'll be placed in a war if they leave. They also don't seem to honor alliances. Yet again, one of my allies decides to leave the UP and because of the first thing I put in that means I was forced to declare war on them. And I get treated as the aggressor and backstabber... like wtf?
Two simple things that could improve this mechanic:
1. Leaving the UP shouldn't cancel the vote. Let's say the leader wants you to pay a huge tax to everyone else. Or to prevent you from researching military techs or to stop your war. You want no part of that. So you leave. You don't have to do any of those things anymore. It's no longer your problem. So what does it matter if the rest of the UP are handicapping each other with those demands?
2. Alliances should honor each others votes and vote for the same thing. To prevent messy situations like them being forced into wars with each other.