I don't really agree with that in all contexts. I think for just urban operations in general, something like a carbine is ideal, but I think for the specific context of defensive urban operations, the shotgun has some pretty major advantages. For one, it certainly has shorter target acquisition times as there's no real aiming per se. If someone is breaching in on you, you simply need to point at the breach point and fire whereas the fireteam which is breaching is going to need to identify your location upon entry before firing. Likewise, most nation's room clearing tactics advocate tight stacks entering in through a single breach point followed by moving to points of domination within the room. This means that if the defender is using a shotgun, there are fairly high odds that even one shell might actually kill or incapacitate multiple team members in the stack since they're basically on top of each other. Obviously, if the one and two man become casualties immediately upon entry, the odds of the three and four man getting hit are substantially higher since now they no longer have the element of surprise and they're going to have to push past the now dead/injured one and two man. This fact is actually one of the things which made shotguns so deadly in trench warfare during the first world war. Because you have tightly grouped targets, more than likely a single shot is going to score multiple hits and in that close of quarters, incapacitation is most likely going to result in death. Another thing to consider is the fact that most countries use some form of body armor now. An intermediate round like what you see in most armies is not going to reliably penetrate. While buckshot certainly won't penetrate either, if you point and fire center-mass, more than likely at least some of the shot is going to land in locations not protected by the plate. Namely, the neck and face, which will mostly be lethal, or the shoulders and arms which will be incapacitating.