I wont agree with this. Unity could also have supporting open world as unreal, at the end it depends of how you implement the tools. You could also waste lots of memory with Unreal when not using there scene persitence system correctly. We made an open world game in unity using our own streaming controller technic and a small tool. This took 2 days for working and is as simple as Unreals approach.
Then we could also include GameMaker, because with the appropriate knowledge you could also turn it into a 3D openworld engine. :lol:
I started to make a list of the advantages Unreal has over Unity and had to stop, it's a long list.
In short Unreal will give you about 20% to 60% more static performance, 20% to 40% Animated performance depending on how skilled your artist are. Meaning you will have a larger and more populated world.
Unreal has most of the stuff you need, there is no such thing as perfect engine that can make any game, Unreal is as good as it gets.
Want to allow players to change the colors of the character? In unreal you just use a texture mask and a few parameters, in Unity you need to make your own shader to do this.
Want to make a working door that can be reused and visually changed? In Unreal just make a blueprint for the door, all components are interchangeable, in Unity you have to make your own script for this.
In fact making things in Unreal is so easy, your artist could even make fully operating game blueprints for level designers to use.
The largest differences between Unreal and Unity is that out of the box Unreal has all the tools you need for large scale openworld games, in Unity you have to make it yourself or find a addon for it; often you have to pay for these addons.
It's not that Unity doesn't have advantages over Unreal, it's just the advantages it has is often more of a personal preference.
I'd normally stand by Unreal. But if you don't mind a buggy editor that has performance problems, not the most intuitive UI, and will crash frequently, Cryengine would honestly be your best bet. Why? because it was initially made and intended for the creation of games with massive worlds.
Cryengine does allow for a larger static world than Unreal, in animation it's about the same. Cryengine and Lumberyard have all the tools also, however the interface is a high contestant for the worst interface ever.
Lumberyard has lots of powerful tools, it's packed to the brim with it, a lot of it isn't connected. That is like getting a sports car - for free- but you have to assemble it, a nice deal if there wasn't a ready to drive sport car; for free with terms.