If someone has strong WebGL, ThreeJS experience how easy is the transition to BabylonJS?
I think you are going to have state whether you are talking about moving a code base, or picking it up for new stuff. The answer is probably going to be much easier to determine if just for new stuff.
Moving a custom webgl code base to BJS is almost unanswerable. It could be easy, or even impossible.
More referring to if someone is a seasoned developer with strong WebGL and 3D development background how difficult would it be to pick up BabylonJS as a developer.
its ezpz, there is lots of resources and active members to guide them.