I saw a comment on Paradox's forums that almost every major patch breaks vassalization and other aspects of war goals / CBs so I wouldn't be surprised if they did break it again.
So far I haven't gotten into any wars in 3.0 yet, but I do really like almost all of the changes so far.
1. First contact is a lot better overall. It's more fleshed out to begin with, but it also now takes up your envoys' time instead of scientists, meaning you don't have to spend 6+ months of society research on every encounter now. I always avoided the projects before and just waited for the other empires to contact me because I didn't want to lose the time. Now you don't have to.
2. Projects and archaeology sites now give way more research as rewards. This is great because it means that the previously very steep initial research costs are essentially halved during the early game since you have so much stored research sitting around, making the tech costs feel a little more natural in progression.
3. Industrial districts are very nice. I don't know about most play styles, but it makes the game so much less frustrating when playing as a life seeded empire early on, who normally have ridiculous consumer goods requirements due to 0% habitability everywhere. Now you don't have to wait for a building slot to unlock to make more, and it also makes it easy to maintain alloy production early on for the same reason. Life seeded is a challenge origin so it's not a great metric to measure by, but I expect it makes things easier for everyone early on.
Other things will take some getting used to, like having fewer building slots but building slots that are easier to unlock. The empire wide population growth penalty is also really dumb, but I know it was put there for performance reasons so I'll wait until I get to the end game and see how it feels before I complain too much.