IMO the back is easily the worst possible place for the sensor out of the 3. It's natural yes, but only accessible when you're holding the phone normally. Completely blocked when the device is laying on its back, awkward at best when it's mounted. Also not very accessible while holding the phone sideways.
The side is a little bit better. Natural enough to use while holding the thing normally, can still be operated otherwise if a bit awkwardly. Can use all 5 of your fingers.
Front is easily the best. Basically guaranteed to be accessible in any situation where you need to unlock the screen. Very natural to touch for the thumb while holding the phone normally, easy to use with any other finger while it's not in your hands. Doesn't require any extra holes or gaps in the case, if it's a modern in-screen sensor, which are more than fast and accurate enough these days.
What's the death count at those events though? Did any of them require major construction to restore the damage done to critical public infrastructure?