Data Layers have been incomplete for a long time, with many necessary functions only available in the Editor Subsystem and many more not available anywhere. For example, there is no way to load data layers asynchronously in the background. That means that if you want for assets to load gradually in the background, it won’t happen. Instead the game will stall and block the whole main thread if your machine can’t keep up.
We also looked up how Valley of the Ancient handled loading/activating Data Layers and how it signaled when the process was finished. Basically, the demo puts everything on pause (with a fade to white screen) while the loading happens. When it’s done, it fades out of white. Not a very robust feature.
This Data Layer solution was advertised nearly 3 years ago as a mature replacement for Level Streaming in an Open World concept, yet all these years later it is still technically less capable than Unreal Engine 4’s Level Streaming functions.