Both a friend and I have experienced a crash to desktop without warning during various stages of gameplay, whether in multiplayer or not. There is no error, no warning, and the game can be immediately booted back up but all multiplayer content/progress is lost. In singleplayer it just reverts to the nearest autosave.
The primary point of frustration is that it can happen at any time. We were one turn from finishing a 45-minute match and we both experienced the crash within a minute of one-another.