Yeah, I'm not sure how much was used final shots though. If you had character heavy backgrounds, I'm not sure Unreal would cope with that compared to just using it for backgrounds.
My instinct is it might have been used for long focal length shots where the background is slightly out of focus going by Jon Favreau's comments here.
https://youtu.be/apLzZBqfqeU?t=4697I think it's of a combination of approaches, some totally in camera, some just used for lighting/reflections, sometimes just used to bring in a green screen if they want to (shown on the unreal behind the scenes video)
in the same way they were giving the impression that BB-8 was mostly remote controlled, where in reality it was puppeteered for a lot of shots.
It's still impressive, and definitely a game changer, but I can imagine there are a few things that need to be sorted out - shooting with a b-cam would be a bit tricky - what the limits of slow motion are, would shaky cam moves be a real distraction for performers (as the background is locked to the camera)