For sure, if you restrict what can (and can't) be done in shaders you can optimize buffer layouts and rendering passes, not allow things that explode shader permutations/switching or explode shader instruction count and kill occupancy.
The flip side is the artists loose control and cannot author custom materials.
I think it can actually be a bit better in terms of artistic consistency to have a fixed set of materials you can work with in a given game. Very important if you want any sort of stylized rendering!
The flip side is the artists loose control and cannot author custom materials.