I know that i can do it in way you described, but i'm not experienced much with creating textures. I'm skilled with creating complex material in Unreal Material editor. I just simply chosed way easier for me . I don't think that this material is heavy. Material are very often very light for engine. My material network is based mainly on simple math operations (like adding inputs together, or multiplying by themselves) and blending operations.
I was thinking about it for few days
It looks empty because meat saws machines don't have mechanism and cables yet. Secret area don't have also some small detail models (like small pipes, or entrance isn't boarded up [in this place i used google translate and i'm not fully convinced about translation] )