Dr. Fackler has worked with most small arms ammo, to include rifle and shotgun ammo. His recommendations on shotgun loads do seem to carry some weight.
However, to put things in perspective he is usually recommending ammo for Law Enforcement use. This is also the basis for FBI standards on ammunition and penetration. This is where the 12" minimum comes from for the FBI.
Bear in mind that in law enforcement they are taking many scenarios into mind. This includes operating in multiple climates where layered clothing can make a big difference in how ammo performs. Law enforcement may have to engage targets inside of vehicles, behind barriers, etc and still make adequate penetration.
Just listening to a county medical examiner may not be quite as large a data base as someone who sees data from shootings alll over the country and overseas. A local ME only gets to see the dead patients. So if he sees a guy who took #4 Birdshot to the chest from 6 feet then 100% of the cases he has examined resulted in death. Does he know how long it took for the patient to expire? Does he know exactly how many layers of clothing the patient was wearing? Does he know if the guy took cover? Does he track the people who end up in ICU and make it after they have walked away from a shotgun blast of bird shot at greater than 18 feet, or the ones who had one lung collapse and still have the strength to resist arrest for 20 minutes with the air in the other lung?
I'm not trying to say that birdshot isn't leathal. I'm trying to say that there are so many variables that can come into play that it can make birdshot a less than reliable load to bet your life on.
These are variables each individual has to take into account for their own home and their own defense plan. If you plan on holing up in a room with the familiy and making the bad guy come to you through a "funnel" if they are going to become a threat, then bird shot may be a great choice as long as the plan holds up.
Shotguns are versitile adn you could easily have a "back-up" load of heavier shot to account for a variation in the plan, and you practice load swaps on a regular basis so you can shuck a shell and port load a buckshot load or a slug without looking. Good training.
Accepting the FBI standard may not be the best for someone in an appartment. Your average civilian home owners and apartment dwellers don't have to shoot through cars (Hopefully) to defend the house.
For me it's going to be a buckshot load though. But I have the potential in my hose to make one or two shots that could span 25-30 feet against an armed assailant. My closest neighbors live at least another 20 yds away with all others outside of 50 yds in any direction with fences to also help slow down shot along the way as well as their own walls.
Hitting a guy in the vitals may be a bit more of a challenge with bird shot if he takes a position like this:
Instead of being like this when it's time to shoot:
(That's a number 4, 3" turkey shot load in the belly at 21 feet, by the way)