Spec/gloss works just as well but uses a roughness texture (inverse of gloss). Internally both workflows are used, it's up to creator preference. I don't recommend pure black on roughness, as it makes the highlights too small and bright. On metal this also means the player tinted color will have less spread over the surface.
If you're using Painter, you can download our export presets for both workflows here https://www.warframe.com/steamworkshop/help/index.php?page=substance-painter-shader