James Davis, Jr. wrote:
Learn ukemi on mats, then practice on hard floor. In that order.
It's an eye opener.
We always train on mats and I remember that as beginners we were discouraged by our sensei to try it on hard floor for the first couple of months (most of us did it anyway
Later, when we felt more and more comfortable with it (and our egos and bravado started kicking in), sensei proposed we all try it at home on wooden floor. "That'll set you back on the ground."
Nowadays I do it sometimes at home after morning stretching excercises. Quite comfortably, I might add.