Hey there! I only recently got a bit of a deeper dive into Solaris, Karma and Material X.
I'm working on a look development for these tiny little jumping spiders that have iridescent scales. For the scales I'm using the MaterialX Standard Surface. I managed to gain fairly good control over the thin film parameters by region, facing ratio, different masking and lerping operations, the shader behaves very nicely.

I figured I needed to have a metallic look to these scales, since the referenced animal under certain facing angles have this appearance on the scales and I think here the shader breaks. I would expect the thin film to work with the metalness turned to 1, but in this case everything goes to black. If I lower the metalness a bit or somewhere between 0-1, I just get a linear darkening as I go towards 1 and maybe a bit of the metallicity effect in the mid range for the parameter.

I thought that maybe this is to do with the MaterialX Standard Surface shader, so I tried to layer the thin film BSDF on top of conductor and dielectric BSDFs in two different occasions and the thin film only seemed to work properly with the dielectric BSDF, but not the conductor. I basically get the same behavior as in the MtlX standard surface.

I'm wondering if this is a known issue, bug or maybe a feature that hasn't been yet added? Tempered steel would be iridescent and there are a range of colors you can get that this thin film describes beautifully if you get the correct thickness and IOR (I find the specular IOR plays a role too), but this is not something achievable at the moment, since it doesn't work as soon as I turn the shader into a conductor. I am using Houdini 20.0.547. Any help would be much appreciated, thank you!

Thin film is only implemented in the dielectric bsdf. Complex IOR with thin film interference is not yet implemented. You can file an RFE to ask for it to be implemented.