It seems like, when the roughnessFactor property is not present in the file (for example Assimp doesn't write this value when it's equal to 1.0), ClayViewer interprets this value to 0.0, when it should be 1.0, in this way models are way too metallic. Can you confirm that?
The value for each property (baseColor, metallic, roughness) can be defined using factors or textures. The metallic and roughness properties are packed together in a single texture called metallicRoughnessTexture. If a texture is not given, all respective texture components within this material model are assumed to have a value of 1.0
It seems like, when the roughnessFactor property is not present in the file (for example Assimp doesn't write this value when it's equal to 1.0), ClayViewer interprets this value to 0.0, when it should be 1.0, in this way models are way too metallic. Can you confirm that?