Why?
Unreal’s PBR system requires specific base color and metallic ranges
Value too dark
weak material response & low light bounce
Value too bright
washed-out highlights & high light bounce
Metal materials
Value too dark
above plus no visible surface reflections
Key PBR Ranges
Materials should generally be either non-metal (0) or metal (1)
Metallic values between can work in transition areas, like raw metal blending into rust.
There’s no single “correct” roughness value—materials vary, often within themselves.
Materials that are fully smooth or rough are flagged to help catch accidental values of 0 or 1, or where more variation might be needed.
Further reading
If you want to know more about Physically Based Rendering, these great resources will help:
The PBR Guide Physically Based by by Anton Palmqvist Physically Based Materials in Unreal Engine
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