I would love to explain what is being done incorrectly and how to improve upon it, but that would require writing an in-depth paper on PBR theory and application, something that I do not feel I should be required to do when a paid service is claiming to be a ‘PBR Texture Library’ and such resources are numerous on the Internet already.
To summarize though:
1.) As mentioned in my first comment, these textures are clearly photo conversions into normal maps (using NDO judging by the results) which means that the normal map detail is not accurate and can not light realistically because it has been converted from photo source and not baked from physical.
2.) Most of the library appears to have lighting information baked into the albedo textures. Since albedo represents the unlit base color only, means that they also can never light correctly.
3.) The specular response on most of the materials is clearly not based off of PBR data. The metals do not look metallic, and almost all of the materials (including rock) have a similar roughness and reflectivity range to various forms of plastic.
Once again, I apologize if I came off a little harsh, but I joined up yesterday expecting big things from the Unreal Marketplace, and was honestly rather let down and disillusioned to see so many materials claiming to be PBR yet missing the mark so dramatically. The concept of PBR is one that is still very confusing to many developers, and it does not help matters when the waters are continually muddied by service providers giving misinformation, or worse, profiting off of those who don’t know the difference yet.
Best,
-Marcus S.