Not Apple engineer here but do have an opinion. Up until this WWDC I would have said RealityKit still isn't up to par with SceneKit, but they seemingly have narrowed the gap this year.
Now with the proper collision detection and exposed skeletal information, you can do pretty much everything SceneKit can do.
That said, if you plan on using GamePlayKit, you will probably still want to use SceneKit as it would get awkward and might not really be possible to get good results with RealityKit.
To get RealityKit to not function as AR, all you have to do is set the ARView camera mode to .nonAR:
https://developer.apple.com/documentation/realitykit/arview/cameramode-swift.enum
You now also have the option of utilizing RealityView directly in SwiftUI.
Topic:
Graphics & Games
SubTopic:
RealityKit
Tags: