While we are using the Unreal engine, the server back-end has had to change to prevent a lot of the hacking that goes on when you run server and client on the same machine.

As for player generated content, while it would be possible for Unreal experienced people to create, there are other issues to consider - Like serving the content across all of our players, checking and approving the content, and other content development issues. The volunteer programs that run events and change day-to-day content has never had to deal with adding large patches to getting their work into the game.

-InQWis