It's way more complicated than this and there are almost unlimited factors involved which is why no one can say with certainly that boosters or harnessing is safer for a toddler. There is no testing of harnessing vs. booster because it's too subjective, too expensive, and also meaningless. Both are approved and safe. One might be a little better, it's up to you what you want to believe.
I have watched lots of crash footage and regularly speak to people at a crash test facility and other people who have been doing car seat research for 30+ years.
If your seatbelt fails, which is unusual but does happen, there will be a disaster regardless of seat.
If you are in a belt positioning booster and your seatbelt fails, there's nothing left to hold you in and you become a projectile. If you are in a harnessed seat that is belted and tethered, unless the tether, belt and harness all fail, you will at least remain in the seat. A tether is not meant to restrain a seat, but it can prevent it from becoming a projectile. THe odds are in your favour that you will at least not be ejected as easily in a harnessed seat even if the belt fails.