Buckshot isn't about making up for poor accuracy, it's about creating multiple wounds to create more tissue damage with greater blood loss in a shorter period of time. The fact is that to get the most from it you need better than average accuracy to make sure all the pellets hit. "Experts" have dissed multiple projectile loads in handguns for a long time due to the fact that they are hard to hit a bullseye with, but that isn't what they are meant for.
The problem with handguns chambered to fire shotshells is that the powders used are for longer barrels and there is a substantial loss in velocity when shot in short barrels. I know the popularity in these type of firearms are growing due to the ammunition that is being offered for them, but I don't know if anyone is addressing the powder/velocity issue. Some of it is made along the same lines as the old "buck and ball" load that was used during the revolutionary war.
As others have mentioned the guns are large and heavy and that is a major consideration for carrying them all day, concealed or not. If I had one I might use it for self defense around the house, with handloads only, but lugging one around is out of the question.