The Uranium cubes don't tell you anything - they just didn't have enough, or enough heavy water, to achieve criticality for a nuclear reactor.

The real goal was to build a nuclear bomb, not a reactor, and the main reason the Germans failed at that is different - Heisenberg incorrectly calculated the critical mass of U-235, thinking that tons would be necessary, which would be impossible to produce (unlike a heavy water reactor which doesn't need refined U-235).

Of course even if Heisenberg had calculated correctly, it's not clear that Germany had the resources or mindset for a Manhatten project scale effort to actually pursue it.

One interesting stat on the Manhatten project is that is was using 10-15% of all electricity generated in the US - just to refine the 64kg of U-325 needed for the Hiroshima bomb.