I agree with Systema style ukemi being useful, but I can't figure how it is better than slapping for when you're being nailed to the ground by nage as in, for instance, judo makikomi throws or wresting high amplitude double leg or suplexes.
Could you ellaborate?
The traditional Aikido breakfall and judo ukemi is taught by having you displace the downward force outward through arms in a slapping motion. The very limited Systema work that I have done made me rethink my approach. I still emphasize a soft body and round shape as a great way of minimizing the "corners" impacting with the ground. By remaining soft (and still connected to the nage), I allow force dispersion to occur throughout most of my body (leaving the head out of that idea of course). Depending how I am approaching a solid surface and the makeup of the solid surface dictates how my body is actually dispersing the force outward. It might look like I am doing the same ukemi when I am doing a sacrifice throw or being taken down, but it sounds different to other people and it feels a lot better to me! Does that help at all?