He’s right I’m currently working on an unannounced open world game created with Unity3D and I can tell you that we have a rather hard time to get it up and running with good performance and low loading times. Since we have no access to source code it’s very hard to optimize stuff for our needs. There are so many things missing here which we could really use in order to build the game that Unreal 4 has support for that we wished that at the time we started development Unreal 4 already announced its payment model.
I guess you could create an open world game with every engine you have source code access to but with some it would be a lot of work. An online multiplayer game even more so. I can tell you that an open world game on it’s own is a very large project and needs a lot of work to be done, but doing this online and dealing with all the hassle like collision detection, ai, cheating, lag compensation / interpolation and this while supporting a rather large amount of player is just so much more work. Eventhough Unreal 4 has built in multiplayer support I guess it won’t be very good for a lot of players (thats what one of the Lead Programmers told me @ GameCom last year anyway) so you would have to write other stuff by yourself like proximity lists in order to prevent too much bandwidth to be spend sending all the state updates to the players around and so on. You could use something like SmartFox as a standalone server solution, but this would confront you with a lot of stuff that is missing (physics, ai, data / asset handling like terrain and so on).
So I would highly advice against using Unity3D for an open world multiplayer game from my own experience because it confronts you with so much more hurdles. Unreal 4 isn’t perfect but has pretty good support for Open World Streaming methods as the others already mentioned + you always have Source Code access so it’s possible to highly adjust the engine to the needs of such an open world online game.