Here's the thing about shotguns as defensive tools:
With handguns and rifles every best practice out there is teaching people to fire multiple shots rapidly. Because that's what you have to do to stop a threat. You can expect it to take multiple rounds from a handgun or a rifle to force the other guy to stop what he's doing.
Convincing someone is different than forcing them.
Those shots have to land to have the intended effect. Which means in the 3-5 rounds you can expect it to take, you have to repeatedly get those shots right under high levels of stress. If you've spent any time behind a handgun or rifle shooting multiple shots at speed you know how easy it is to miss with a number of rounds in a series of shots.
One of the things I do in more advanced shotgun classes is have people test how long it takes them to draw a pistol and put 3 rounds on a B8 sized piece of steel at 10-12 yards. (Red Stitch Dum-Dum targets) It's usually a much longer time than they anticipate because they miss. Sometimes folks go through almost an entire Glock magazine before getting those 3 hits. Some folks we called mercy on after they had to reload their pistol.
Then I have them perform an emergency load with a shotgun and fire one shot and we see how long that takes. So far nobody has missed with the shotgun shot. Not even the people deep into their second magazine with the pistol.
Part of that is the long gun is easier. Part of that is the training: Training them to understand what the shotgun does and how to use it.
When you understand that the shotgun's virtues mean you only have to get it right once, and then you're trained on how to actually get that once efficiently, it has a way of changing how you are using the gun. Because it has that fight stopping power you are able to spend a little more time making the shot count. And a little more time (an extra 1/4 or 1/2 a second) is usually all it takes to get a much more accurate shot on target.
The virtues of the gun multiply when we discuss engaging multiple targets at relatively close range. Firing a series of shots from a pistol or a good rifle on multiple targets takes longer than firing a single shot from the street howitzer on multiple targets. In class I routinely get folks to fire three shots to the high chest of a threat target from a ready position in 3 seconds or less.
It's a rare crew that can press an assault against that kind of response.