It may be that you have no indirect lighting so non-metals are unable to show reflections. Try adding an ambient cubemap or setting “environment color” to non-black in the world settings and try again.
There should not be that big of a difference between metal and non metal.
But one interesting tidbit… when making city backdrops a while back I found when making ‘realistic’ glass that the skyscrapers would not reflect the city brightly enough compared to photos etc. After a bit of research I realized that most commercial skyscrapers don’t just use glass, they have a bunch of specialized layers combined into a film to alter the properties of the window. A common layer is a thin metal layer that actually does act like a metallic reflection layer in ue4. It helps keep the buildings cooler in the sun. So I made the buildings metallic and didn’t worry about it too much after that 