Hello slarti,
So I think you have a misunderstanding of what the ‘Feature Level Switch’ node actually does.
It actually handles switching to the appropriate shading model based on the device, and will output the correct and compatible shader properties allowed for that specific shader model.
Utility Expression
Simply using this node on the materials you want to have checked is all that is needed. Unreal will handle the rest of the work. I will warn you that subsurface shading requires opacity, and opacity on mobile devices is incredibly expensive due to hardware limitations.
Let me know if you have further questions.
Thank you,