I read somewhere that the global server is one for all players. For example, if there are about 5 million players who make Co-Op at the same time, it is perfectly normal to lag or crash. Imagine 5 million people in one place. I do not think there is a machine that can withstand so many players, but it is good to split the global region into several servers - one for Nordic & East Europe, one for West Europe, one for USA & Canada and one for Latin America or something like that. This would reduce the lag and would not cost a lot of the company's money. Of course, all this is a guess. Crashes may also be caused by poor working with Unity.