I've used the caddy tsgb16 and tsgb24 brackets and hotels where the rooms are mirror images of each other and boxes would wind up directly back to back if mounted on the studs. Since we always put in 1900 boxes for low-voltage, this was very little added expense, especially compared to framing out kicks off of the studs ourselves so the boxes would not be directly back to back.
Regarding boxes set in concrete, wouldn't the concrete itself pretty much ensure the box would never fall? I realize you still have 8/32 screws vs 10 - 32 or 24, but I have never seen a metal box with bent ears or screws ripped out from it from the weight of any ceiling fan.