Train on concrete.
I have only ever done simple rolling exercises on concrete, but the hard, pointy, etc., ground is great for learning how to engage/move softly! Studying how to spread or otherwise manipulate incoming force through/around the body safely, and moving around immoveable objects spontaneously and at will, is kind of what the budo aspect is all about it, isn't it? We're rolling along the path and suddenly that little rock you didn't notice makes its point real quick. Your proprioceptive awareness for all the boney bits in your body tends to go up too, I imagine.