Roughness controls the how sharp or diffuse a reflection is.
Specular controls the intensity of the reflection.
Unless you’re making vantablack, you should not be setting your material to 0% specular or 100% roughness. Everything has some amount of both.
The issue with sunlight hitting landscapes at steep angles is really a problem with normal/roughness mipmaps. As mips get higher they have lower resolution and their surface approximation gets worse. It’s perfectly fine to use the distance to control the specular to overcome this problem.
Another which I personally think is better, is to use a normal->roughness composite texture.
