Don’t listen to this guy. From his posting history it seems he only posts to troll the engine in the majority of his posts.
Unreal Engine has been known for being used in many open world games. Only thing is this is out of your scope with what seems to be your level of experience. You will need to edit the engine to do what you need it to. You have full source, so you can make it do whatever you want.
It will definitely be more complex to do, no matter what engine you use you will have to tweak the engine to be able to do what you need it to (or spend money to license middleware that does it for you). This is even true in Unity, which was his suggestion… Only difference is there is more middleware for Unity as it has had the Unity store for years now.
It’s just ironic that he discussed problems with an open world online game, when 1) You never mentioned an online game, and 2) you need middleware for ANY online game in Unity that’s more than the very most basic networking capabilities… and for SURE in an open world online game. There is a reason that even RUST uses uLink (and I am also licensed to uLink myself, having not even made a game nearly as complex as that).