I tried to merge my world (~600 regions) with the main campaign a while ago and did not get it to work. Doing so the game gets unstable (stops loading assets) or eventually crashes on loading in. I think the limit seems to be 1 600 regions per world before that happens, but I’m not sure.
You can import, load and edit all ~600 regions in the editor just fine. Loading a few regions (e.g. 100) in-game works fine too.
Can anyone come up with a trick to get around it? I’m actually completely clueless when it comes to things like this.