I have no issue with "training" but I'll admit that beyond the infrequent trip to the range, my "training" mostly consists of what I learned in basic. I have shot guns since I was a kid, but truth be told, I'd be hard pressed to draw smiley faces with a 9mm at 25 yards right now.
But with a shotgun that somehow doesn't matter as much. Mine comes to my shoulder naturally and the bead settles where I'm looking without really thinking about it. It points and handles well for me. The recoil is pretty stiff with some loads but not at all bad with tactical buckshot, actually probably less that the recoil from an AK. And even with full power loads, I'm a chunky guy in my 20's. I can take it (actually, I sort of like it!). But for those who are recoil sensitive, have arthritis, are smaller statured, or maybe just smarter than me, you'd be just about as well protected with a 20 gauge too. I'm sure this has been brought up before, but if you actually have to use your gun to defend yourself you're almost certainly going to lose it. I like guns and I'd like to have an AR, but it just isn't in the budget. If it were though, two of them at around $1K each wouldn't be. So when the police confiscate your lovely AR carbine after a defensive shooting, you had better be able to replace it or you will be defenseless the next day.
Another $150 for a used pump would be much more bearable to me and many others than $1000 to replace an AR. Plus with an AR, you need mags and accessories. I never really feel comfortable with a gun without having a good supply of ammo for it too and with an AR, that would mean about 1K plus some for range use. With my shotgun I feel comfortable with a .50 cal ammo can full of slugs and buck. Altogther I have about $200 in it INCLUDING the ammo can.
For that price, it gives me pretty much all the capability I need in a HD gun and then some.
Also, I don't know exactly where training came into the discussion (although it is a good point) but I would argue that the less training you got, the better off you are with a shotgun instead of a rifle (probably) or a handgun (almost definitely). Long guns are generally easier to shoot than a handgun and shotguns have been the easiest for me. Like I said, get the bead on it and pull the trigger. About as close to foolproof to me as it gets.