I say that it's unrealistic because it doesn't happen.
As private civilian carriers, all but the wealthy run into a resource problem. We have a finite amount of time to train or practice, and money to throw at ammunition or other linked resources. Consequently, we run into the question of how to best convert our resources into advantage.
Within that limited time, it is ideal to develop a number of distinct skills: shooting accurately with speed at varying distances, draw --> assess shoot/no shoot, draw --> first hit speed, target transitions, reloads, malfunction clearance, precise fire/ called shots, shooting from different positions, moving while shooting, strong hand only, off hand only, shooting movers, etc. We wind up in the position of having to prioritize skills to focus on, and determining how much time we put into each skill. In many cases, we have to determine which skills are sacrificed.
To determine which skills to invest most of our resources in, we need to look at which skills are most critical. That's a matter of determining frequency of relevance. We know that the overwhelming majority of private citizen DGUs are in the 0-7 yards range, increasing in frequency as the distance gets closer to the 0-5 foot range. We know that most resolve with no shots fired. Based on this, it is very clear that rapid deployment from concealment, identifying shoot/no shoot, shooting from retention, shooting while facing forward and moving backward and/or laterally, shooting with speed at extremely close distances, and strong hand only are more likely to come into play than precision fire from 25 yards, speed reloads, shooting from prone, weak hand only, etc. To be clear, these are ALL good skills to have, but some are more likely to matter than others.
It should also be apparent that the most critical skill for civilians is developing a fast and clean draw while identifying shoot/no shoot. This is important to consider, because getting a fast and clean draw is the aspect of DGU that suffers the most from carry rotation and location/method change. Some skills (trigger control, target transition, etc) translate easily across a variety of designs. Draw is not one of those.
In a real event, the defender is doing a LOT during the draw. Here's an example, using a high % situation-- low light, threat is a pair of men who reveal themselves as a threat within 5 feet. During the draw, the defender is...
- locating the firearm
- clearing cover garments
- locking into master grip
- moving to create space, backward or sideways
- yelling to draw attention and to try to 'freeze' the BGs briefly to create time
- presenting into a retention position
- observing BG reactions to defensive movement and introduction of lethal force response. IE, identifying "shoot/no shoot"
- prioritizing targets
- mapping an exit
- identifying what is behind the targets
- trying not to trip while moving
I've been in a very similar situation, except that it was dealing with one positive BG who revealed a firearm and implied intent, and a possible second BG who was physically adjacent to me, with no visible weapon. The situation resolved with no shots fired, as I was able to freeze the other two by yelling, moving, and drawing explosively, and presented lethal force before they could establish control of the situation. Drawing smoothly and quickly, and being able to focus on what was in front of me, either saved my life, or my life as I know it (no legal hang ups).
Having been through it, I get badly agitated when I see people reference Nutnfancy style, multiple pistol, multiple method, multiple location carry rotations. What it speaks to is obvious: complete disconnect from the single most crucial aspect of private citizen DGUs. Many of these guys are often making choices that they justify by saying that "the gun you have on you is better than the one at home". The problem is that the statement doesn't work if you can't get the firearm deployed fast enough to use it. Also, the faster you are able to deploy it, the less likely you are to have to shoot.
Because of the above, my view is that best practice is to carry consistently. Same weapon, same method, same location. With that, become the best you can be at deploying cleanly and rapidly. ALL of my draw practice is with my EDC, and I practice draw, draw --> shoot/no shoot/OODA loop, and speed to first hit at least twice each week for 15 minutes each session, with randomized determining factors and a timer.
As far as shooting from ready goes... Once the firearm is out and ready, most skills translate. Trigger control, recoil management, etc apply the same way (though minute differences in performance will appear between pistols).
Rewinding to the top, we consider scavenging. Likelihood of relevance in a private citizen role is effectively zero. Even in mil use, scavenging isn't active, meaning it's not jump on it and immediately deploy it. Training with enemy systems is brief. It's familiarization of basic function and field strip. A friend of mine spent 8 years with SF, and even his training was predominantly M4 and M9. During his deployment time, all of his actual combat shooting was M4, M9, and a brief experiment with a 1911.