Function-based materials are completely dielectric when using feature level ES2. Any maps plugged into the function’s metallic slot do not seem to count towards the number of mobile texture samplers, either. It’s worth noting that the materials only appear to be dielectric in the actual game world; they look fine in the material preview.
At the moment, I can get around this by using the BreakMaterialAttributes node and plugging everything in manually. See attachment.