I carry SOB and find it works extremely well. I don't use one of the purpose built SOB holsters, but rather have a hybrid IWB rig with the cant adjusted for a smooth draw.
I've never felt that any of the complaints really hold water.
Back Injury: First, how often do you fall onto your back? On the rare occasion I do fall I generally wind up falling forward. Second, I wear my gun more like 5:00, so it's not right over my spine anyway. If I ever get tackled onto my back in an SD situation, a bruised love handle will be pretty low on my list of worries.
Flagging people/long draw: You could flag people if you wanted, but I find my gun is pointed down for the bulk of my draw.You're only talking a few more inches of travel than the typical "behind the hip" holster placement.
Also, regarding the length of the draw, while it may be longer it also looks/feels a lot like reaching for a wallet. For me, it's a very smooth movement - and you know what they say about smoothness in relation to speed. I can still get my gun out and shoot in about 2 seconds (generally about .6 seconds slower than from the hip). It might not be the best spot if you're going to be dueling someone at high noon, but the sacrifice in speed is over blown.
Uncomfortable: Depends entirely on the gun. My XDS I barely notice is there. It definitely isn't a great carry position for long car trips, but - at least with me - there's a natural hollow for the gun to sit in. My shoulders rest on the back of the seat, but my lower back, not so much. I rarely have a problem. Bigger guns do get uncomfortable in the car, but it's easy enough to take them out and put them in the glove box.
Re-holstering: I've never had a great deal of difficulty, but I admittedly don't see it as that huge of a deal anyway. Should I ever need my gun it's going to stay in my hand until I'm 100% sure I don't need it. And then I'll either have time to re-holster (threat gone), or put it on the ground (cops arrive).
Then there's the upside. It's extremely easy to hide a gun behind your back without printing. Your body makes a natural pocket of extra shirt there, and you can carry a surprisingly large gun under even an untucked t-shirt without drawing any notice - particularly if your shoulders are bigger than your waste. You don't have to wear clothes a size too big, or one of those "look at me, I have a gun" photographers vests.
Through most of the summer I wear an untucked, but appropriately sized button down shirt, and even a G19 sized gun is invisible unless I bend over sharply.
So my advise is if he likes SOB go for it.
Of course you may not want to listen to my advise - I've also been known to wear a shoulder rig, so clearly I have no regard for safety
