Man, I love Graphic design. And even more so I love games that just ooze graphic design :D Like Monument Valley. Just recently I watched a GDC Talk about the art-style of Monument Valley. And that got me some key insights!
Needless to say that I felt VERY compelled to create/recreate a shader that emulates the style of Monument Valley while giving the artists a great degree of user flexibility.
The shader works with all meshes, regardless of if they have UV or no UV's at all. It lights meshes with flat colors based on three or 6 cardinal directions (X, Y, Z,-X, -Y, -Z) while providing a lot of options, like worldspace Y gradient shading, rim lighting, light and shadow boosting and colored shadow maps to allow for a maximum of artist control.
I totally looks best in ortographic views, but it absolutely works in perspective views, too. If you still want to do UVs for things like baked in shadows... man, just go for it. The shader supports tinted shadow textures.
The shot above was created with 100% Unity 2017 resources. A custom post processing stack, this shader and a few lowpoly meshes. Nothing else (not even a light :D). There you go. If you fancy this clean graphic look for your game, this shader might be able to help you out. And if you want to change it, go ahead. Or, if you own Shader-Forge, edit it in Shader-Forge :D