This may be a matter of semantics. To me, a “valid defense” is a defense that always prevails—for example, that truth is a defense to a defamation claim in U.S. law. The rest, I call “possible defenses.”

Another possible definition, one you may have meant, is that there's a set of "invalid defenses" that could not possibly work, and the remainder are "valid" even if they might not prevail.

The courts are not deterministic. There will never be a universally valid defense, because crimes are not fungible. There can always be circumstances that change how the facts are interpreted.