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.
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.