What you are talking about is ralated to the texel density of the UVs of the texture used to blend metallic and non-metallic parts. And it appear also in specular/glossiness workflow, it's just less noticeable coz the effect is reversed (black instead of white). It's also highly dependant of the resolution, which is mostly a real-time problem, as texel density. Many offline renders use the metallic/roughness workflow with no issue so i don't know how it could be considered "inferior".