SOB carry has several big draw-backs.
1) Injury: You are placing a bulky metal object against your spine, or kidneys. A fall onto, or blow against, that gun almost certainly will cause more pain and injury than it otherwise would have. If the fall is hard enough, it could do permanent damage. A gun on your hip (3:30-4:00 position) doesn't cause that concern.
2) Discomfort: If you sit at a desk, or drive anywhere, that gun is going to be very painful to sit on for long.
3) Printing: If you always stand straight up, you may conceal your gun well this way. The moment you sit on a bench or other backless seat, squat, bend, twist, reach for something, etc. the small of your back stretches your clothing tight in that area. You WILL print, and print worse than any other location you might carry. If you carry under a jacket or un-tucked shirt, SOB carry causes the outer garment to hang up on the gun and many folks who've tried it found that their jacket would creep up and begin to tuck itself in behind the gun as they moved around -- eventually leaving the gun completely exposed with the cover garment bunched up on top of it or even behind it.
4) Draw: Drawing from an SOB holster invites a tremendous safety violation. It is difficult to execute a draw that does not encourage/require you to sweep your own waist/hips/kidneys/pelvic girdle with the muzzle as you move the gun around in front of you. There are two types of SOB holster -- one butt-forward, one butt-back like an exaggerated OWB holster. The butt-forward version is much worse for this as you're trying to rotate the gun as well as draw it forward. Many ranges and trainers do not allow SOBs just for this reason. When you're trying to grab your gun, disengage the safety, and get on target for a fast shot, no portion of that operation should put your own body in front of the muzzle!
(Of course, no operation EVER should put part of your body in front of your own loaded gun ... but especially grabbing for a defensive sidearm in a moment of panic.)