I just saw this presentation at the Chaos Computer Club conference, for an “Ethical Hardware Kit with a PCB microcontroller made of wild clay retrieved from the forest in Austria and fired on a bonfire. Our conductive tracks use urban-mined silver and all components are re-used from old electronic devices”. It was part of the feminist hardware strand!
There’s also electrical properties, such as capacitive coupling between the layers. FR4 circuit boards have known properties that electrical engineers can use in cases where it matters. RF design, in particular.
This isn’t insurmountable, of course, and there’s plenty of designs where it doesn’t matter much. These sorts of things have to start somewhere.