Not a "techy type" but I do have a few decades of experience. Basically what happens is that dropping a round into a revolver chamber is gentle, and only gravity assists.
Now, when you do that in an auto pistol, it is the same thing, until the slide closes. Now, some pistol designs allow for the extractor to flex or pivot to snap over the rim of the case. Others do not.
And, closing the slide has complications also, depending on the manner used. If you ease the slide forward, and the extractor doesn't snap over the rim easily, then the slide may not go fully into battery.
Conversely, allowing the slide to slam forward on the round has risks as well, dependant on the specific design of the pistol. If the extractor isn't designed (or modified) to snap over the rim, you risk breakage. If the firing pin isn't designed to be held, or if the sping holding it is too weak, it could slam forward and fire the round. The slide moves significantly faster when it isn't pushing a round into the chamber, and what is safe when loading a round may not be if the slide is let slam shut on a chambered round. Also, even if there is no risk of firing, the slide slamming shut may drive the round too deeply into the chamber, with the issues that causes...
It depends on both the design, and the condition of the gun, what is, or is not safe. Loading from the magazine is the intended method for all the autos I know of (and I know quite a few). Some of them will allow safely single loading without risk, others will not. I have one semi auto where the manual specifically forbids letting it slam shut on a chambered round.
As to the risk of bullet setback in am auto pistol, this does not happen with properly constructed ammunition. Even when the round is fed over and over again. With correct neck tension, the bullets doesn not get pushed back.
HOWEVER, not all rounds are built to that rigorous standard. And even rounds from the big name ammo companies can escape the proper QC, and have enough neck tension to be loaded once in perfect safety but suffer setback from repeated feedings. It does happen.
Making rounds well enough to survive multiple chamberings is quite do able, but might not be done, as it is always a cost/benefit thing. I have personally seen a single .45acp round chambered thousands of times over a couple decades, to the point where the nickle plated case had brass streaks all over it from the nickle wearing through. That bullet never moved. And when the owner finally did fire it, it fed and fired perfectly. It can be done. The fact that not all ammo is made to this high standard does not mean that some isn't, even today.
And a good reloader can make their ammo perform just as well.
All else being equal (and it almost never is) bigger bullets tend to work better.