These aren't alternatives, they're complementary. I appreciate that fuzz testing makes sense over writing unit tests for weird edge cases, but "these parameters can't be zero" isn't an edge case, it's part of the basic design. Here's an example of what X9.62 says:

> If r’ is not an integer in the interval [1, n-1], then reject the signature.

Let's write a unit test to check say, zero here. Can we also use fuzz testing? Sure, why not. But lines like this ought to scream out for a unit test.