As a Roblox developer, it is currently not possible to use SurfaceAppearace on Parts. To make a simple rectangle with SurfaceAppearance, you have to go into Blender, export a cube mesh, upload it to Roblox, and apply the SurfaceAppearance, which seems pointless and unneccessary when Parts already exist. Same goes for spheres, cylinders, wedges, etc. because they already exist in the engine.
If Roblox is able to address this issue, it would improve my development experience because it would be easier to build with high-quality PBR textures and use fewer meshes in games.