Interesting information, I would like to know what led them to changing the engine. I remember OpenMW went through a similar situation, they switched from Ogre3D to OpenSceneGraph because of some limitations of Ogre. Am also curious about the bs framework and how it is different from e.g. Godot.
Why does bs use a custom shading language? I hate that! Stick with standard GLSL or HLSL. SPIR-V compilers are supposed to make it possible to take your pick. More vendor lock-in bs, just like Godot. Also no legacy GL 3 support or WebGL renderer. Dude. No.