Part of the difficulty is ...

PoserSurface uses FireFly renderer, which "fakes" a lot of things like specular and metal. It's not a physically-based renderer like SuperFly/Cycles is.  Two totally different methods of rendering.

PoserSurface uses specular, gloss, and reflection maps to create shine and reflection.

Physical Surface and Cycles/Principled uses roughness and metallic maps to create shine and reflection.  Roughness maps work opposite to specular. Darker areas are more "shiny" or "glossy", and lighter areas are more "flat." It's a little more involved than that, but that is the gist of it.

Metallic maps ... white areas are metallic, black are not. Gray somewhere in between.

Once you get the hang of PBR it is SO MUCH EASIER than dealing with a gazillion nodes chained together with spaghetti noodies (which is the major part of the difficulty in converting FireFly shaders that are node-heavy to Physical Surface). I havne't looked back since Poser started using PBR. All the work is done with the textures, as long as they are connected right.