I've been using the Unity3d to Babylon exporter and it's working great. Awesome job! I have a problem that I was hoping you could help me with though. When I export to Babylon with, it looks like the metalness doesn't translate. For example, if I create a sphere with Metallic at 1 and smoothness at 1 (or roughness at 0), I get black in Bablylon. I've tried Standard, Standard Roughness Setup, and Standard Specular setup and they all look the same. I've tried an HDR skybox too. Below are some screenshots. You can see that the sphere on the right is visibile in Babylon, but it has a Metallic & smoothness value of 0.
By the way, I'm using Unity 2017.3.