Sure will be a lot of opinions on one side or the other. In the 80's I worked with a retired town cop, long-gone now. His very- simplistic attitude was that it depended on a lot of things and a 50-50 chance. He had seen death at both sides. I witnessed one where a very young child was strapped in a rear-facing car seat in the back. That part of the car got crushed. The EMT's said if the child had not been strapped, it would've at least been thrown to a safer part of the car and not crushed to death, but who knows at what other injury. A friend of mine burned to death in a (minor) tractor-trailer wreck, witnesses said he appeared unhurt and was struggling frantically to remove his belt when the flames spread to the cab. He had a young wife and two little kids. I know others who were saved by a belt, no doubt about it.
The answer lies in the type of vehicle, the circumstance of the crash, and luck, or unluck. Hit a cow 40 years ago and would've been much better with my lap belt on. HOWEVER, also back then I was a passenger in a 20mph CJ rollover. With the jeep upside down above me like a harmless tent (thank you rollbar!), I was sitting upright next to a big rock which was now smack dab in the free space below my inverted seat. Had I been wearing the lap belt, my head woulda been a smashed pumpkin and you wouldn't be reading this. That one was a REAL eye-opener. Of course there are situations where belts save lives, they surely saved my butt on the race track. But not wearing one in that jeep saved mine that day. I also refuse to wear a belt in any 6x6 on dangerous road or terrain, I feel I have a better chance without it. Whether or not I belt in a normal vehicle depends on location, traffic, road conditions, and speed.
I experienced both sides of the issue and lived to tell,, so far. I will never allow a law to dictate what was and still should be personal free choice. I choose to practice both sides on an almost-daily basis, just as I have done since 1974. Just because more people are reportedly saved by belts doesn't make it acceptable my friend burned alive because of one. There was no seatbelt law at the time, he just always wore one. A belt had saved him before. But not the 2nd time around. There is only luck and unluck.
I found the pics, attached. M-series 6x6 upside down. I do not want to be the one belted and crushed in that cab. I would rather take my chances for all possible outcomes.
CMPPhil above mentioned miltary veh studies. I do know of two, which mostly blame m939-series brakes, but a fatal rollover can happen with any tactical mil vehicle, pre-1990's. Exception, M151 hardtops. In the 1970's six of us were sent to flip an inverted jeep back onto its wheels by hand, and driven away. Occupants banged up some, but no damage to the top. Driver hit the brakes hard and swerved. Not a smart thing with a Mutt. Someone ran a stop light on him.
Here is one study:
https://www.globalsecurity.org/military/library/report/gao/nsiad-99-082.htm
another here:
https://www.gao.gov/assets/230/227104.pdf