I have worn a small of back holster before; carried a S&W 3913. All of the other comments regarding access, covering garment, etc., are all relevant, but what caused me to stop wearing it was not the fear of a fall and injury, but it was the the extreme discomfort. Whenever you sit, you have to shift position to keep the pressure off of your back, whether it's in a vehicle or a chair. Depending on the vehicle you were entering/exiting, it could be an exercise in gymnastics. About the only time it didn't matter on how you sat was on a motorcycle, but then the covering garment argument starts again.
If you're standing in place, like on a security detail, it's as good as any holster, and for me, it was just as fast to draw as my normal strong side carry. In fact, if you're standing against some type of backdrop (wall, fence, draperies, etc.) you could actually have your hand on the gun and it would be less obvious than a strong side draw.
I won't say don't use one, but it's not the perfect one-size fits all holster, despite what all of the TV cops do.