Siavash Tehrani

Nicholaswillis
I know I can accomplish this sort of layering with a single surface shader by just mixing multiple sets of texture channels using masks instead of mixing surfaces

This is what you'll have to do for the time being. The devs have said they'd like to tackle mixing more than two Standard Surface materials with XPU, but that it won't be a quick and easy thing to pull off (paraphrasing here).

Bummer, I figured this was eventually going to happen. Thanks for the heads up and links.

What protozoan said. There's also some sketchy stuff going on with the glass shading in the Karma render.