Yes, you can do that in Unity, but dont forget: The Unity terrain is oldschool, no advanced shaders, trees and terrain dont use bump mapping. And if you paint a few variations dense enough you will get into performance trouble easily.

Your example to paint stones just like grass works indeed, but you cannot layer it on top of each other.

So when S2 handles this with better performance, it could be a winner. But I would like to see it in action to judge this.