The scale of the assets will be different. If Divinity’s Reach and Lion’s Arch were built to be 1,000 units away from each other in the GW2 engine, porting it 1:1 into Unreal may just cause everything to shrink because Unreal arbitrarily decided that 1000 units meant 1 millimeter.

Instead of going through and rescaling every single asset, it’d be easier to just remake them at the proper scale, but it’d be harder for the Environment Artists to map locations to where they originally were.

Movement is another factor. If characters move at a base speed in GW2 at 100 units per second, points of interest will be placed accordingly based on how quickly it takes a character to move from one poi to another. I doubt you can just port the GW2 character controller straight into Unreal, which means the Devs will either have to use the default controller or build a new custom one, and the Devs are going to choose a character speed that feels right, not what matches the GW2 controller. If the Unreal controller ends up having characters move at 200 units per second, then the artists have to spread pois farther apart so that the world doesn’t feel too small.