It's not "unquestionably". I question that statement.
There are a lot of factors that go into what's going to be most effective
in a given situation. The .357 Magnum 125-gr from a 3 to 5" barrel at reasonable ranges- with the right bullet and velocity- should be plenty effective though. Other calibers/bullet combinations that should also be effective (assuming you have chosen the correct velocity and bullet combination):
9x23mm 124-gr HP
9x23mm 125-gr JSP
.40 S&W 135-gr JHP
.40 S&W 155-gr JHP
.40 S&W 165-gr JHP
10mm 135-gr JHP
10mm 155-gr JHP
10mm 165-gr JHP
10mm 180-gr JHP
.41 Magnum 180-gr JHP
.41 Magnum 200-gr JHP
.41 Magnum 210-gr SJHP
.44 Special 180-gr JHP
.44 Special 200-gr JHP
.44 Special 210-gr SJHP
.45 ACP 185-gr JHP
.45 ACP 200-gr JHP
.45 ACP 230-gr JHP
.45 (Long) Colt 200-gr JHP
.45 (Long) Colt 230-gr JHP
The truth is, when you choose a quality bullet at a reasonable velocity, I would feel perfectly comfortable with
any of the combinations I've listed. Hell, in the last few years, I've frequently just had a 9x19mm around as my "get to my rifle" home piece or CCW. (Of course, I've almost as frequently had something like a S&W 629 loaded with
.44 Special JHP near the bed or IWB.
)
Bullets are designed for different missions. There are same-weight bullets that open up or fragment much more dramatically at the same velocity, and there are some manufacturers who tend to load their ammunition down. The shooter needs to do the research, perhaps even just go test the rounds he's considering carrying. I know from my own testing that bullet performance is much different from a 3" barrel than a 4.5. A very effective bullet from a 3" barrel may be traveling too fast, and not penetrate adequately if fired at a close target from a 6" barrel. Some of this can be resolved with modern "controlled expansion" bullet designs (bullets like Gold Dot come to mind), but some of these, in turn, will open very little or not at all if fired from a short barrel or when not loaded "hot" enough.
Most important of all: maintain situational awareness, know when to run and when to shoot, and be able to hit your target.
John