We play coop and have used dedicated and local servers. On the computer running the server either way Steam hangs waiting for sync when player exits game and then tries to exit Steam. You can wait forever. This is handled by just forcing an exit. But...

We had this problem where the game saves never stuck. We'd play for a couple hours, quit and when we come back the world is as if nothing had happened. We tried a few times. To fix this I had to delete the world save and replace with the .old save.

I think this problem occurs when the game is restarted while Steam is still in 'waiting to sync' mode. Not sure. But its my guess.