For the deterrent effect, i.e. a blinding wound, of blasting someone with #12 birdshot the .410 + short-barrel concept works well. Very well. In fact, the designers' original intent was that: One snapshot at a car-jacker's face and get the hell outta dodge.
As for the 'serious' defensive load, unfortunately, the buckshot makes an ineffective killer. The ballistics suck.
http://www.410handguns.com/410_gel_results.html
The pellets must penetrate an attacker's body deeply enough to be able to pass through a vital cardiovascular structure to cause rapid fatal hemorrhage to quickly deprive the brain of oxygenated blood needed to maintain consciousness. And that, ever since the FBI's first wound-ballistics meeting in 1987, is mandated as a minimum of 12 inches of penetration in test-gelatin.
The .410 + short-barrel combination fails the test, and can't be relied upon for effective self-defense outside a very specific set of circumstances.