Their is some versions of Blender 3.0 that has problems with other versions RGB node. The latest version, which at this time is 3.1.2 works with my shaders.

If you are using older version tab into the shader and just replace the rgb node with an rgb node that will be generated by that version and that should solve the problem. 

This happens when you are looking at an asset file that is not linked in your settings. 

The quickest way to solve this is by making sure you save the file in the same location you have your asset library saved. Blender Preferences > File Paths > Asset Library

If you like to save different assets in different location you can also add more paths in the Blender Preferences settings.

Automotive flakes look like noise to AI so it tries to smooth it out. So AI denoising as of 3.0 will blur out the flakes. Hopefully as the AI's get smarter, the denoising will know they are metallic flakes.

It depends on how much contrast their is in the environment. A night scene with many  small lights everywhere will take atleast 4000 samples to start looking good. All the images and videos you see were with 4000 Samples. 

A day time shot with overcast will look good almost immediately 

Yes it does but it is very intense and will slow down your frame right considerably. On the positive side even with small amount of samples it still looks very good in still shots. With animations more samples is needed.

No, its is all Procedural, all my own data is CAD so everything is procedural so I don't deal with UVs. 

This will have to do with your 3d models Face normal direction. Make sure your normals are all facing the same direction in the modeling software you used to model. To flip the normal inside of blender, go in edit mode, under mesh> Normal> Flip Normal

The shader works with the objects scale so remember to check that the scale is set to 1, if the scale is different, select the object and press ctrl+A to apply the scale