The real problem with small of the back (sob) is spinal damage if you fall back on it. All it takes is that once.
Yeah, I've heard that one too. You know, I'm not going to go so far as saying that this statement is false, but it's one of those things that I have a strong suspicion is a "heard it from a guy who heard it from a guy who heard it from a guy" type things, rather than something that is very likely to happen. It's one of those things that everybody seems to know as gospel truth, and yet have a hell of a time coming up with supporting evidence for, like names or dates of incidents. For a start, having spoken with the folks at Bianchi and Galco, neither has ever faced any kind of a lawsuit because somebody was paralyzed by falling on their SOB carry (despite several claims by various people on the internet that SOB holsters were dropped by manufacturers because of "all the lawsuits") and instead discontinued them because other patterns were selling better.
Now, it's absolutely possible to break your back in a fall, but that's hardly limited to whether you have a gun or knife to land on. For that matter, falling sideways onto a strong-side holster that holds your hip up off the ground when the rest of you body hits would be every bit as likely to break your spine--actually more likely because the muscles which support your back are actually much stronger going forward/back than they are side to side, and the spine itself is capable of much greater articulation forward and backwards than it is side to side. Finally, the fact that on no less than three occasions I've actually fallen on top of my gun in the six o'clock position (once on flat ice, once off of icy stairs and once due to a nice upper-cut to my jaw) and never did anything more than bounce. That time off the stairs, I actually landed ENTIRELY on the gun as my hands and feet were in the air, and I'd fallen about two or three feet. I ended up with a kind of funny, gun-shaped black bruise on my waistline, but Advil and a couple of days cured it. True, I'm fairly hale and hardy but if the SOB carry position was as instant-death as many portray it as being, I think I'd likely have at least some numb fingers by now, or something. Not saying it can't happen, not saying it's never happened, but just that--like so many things--the story keeps getting better with repetition. I wouldn't IWB SOB a knife, though---just because it's too far to reach and 4-5 o'clock works so well. I only carry the gun at six because it prints anywhere else.