Sorry to bring up an old thread, but if this is the first result in Google, and the fact that this has over 1600 views, this needs to be addressed. Unity is one of the worst game engines for open world games. In Unreal 4 you can setup a dedicated server right out of the box, create advanced LODs, level streaming, and culling in a snap. In Unity you’d need to coddle all of that together after spending a very long time learning the low level parts of the Unity engine.
You can read a real developer stating this right here - ://www.reddit/r/gamedev/comments/28l37c/one_year_ago_i_quit_my_day_job_and_launched_a/cibz9ak
This quoted post should either be added to by a moderator with an Unreal 4 developer’s statement or removed from Google’s first search result.