Here is my uneducated deduction:
1) In situations where if you fall off you will die; use harness.
2) The most likely situation where you would fall off and die is when alone on deck (in any weather), far enough off shore that you cannot swim to shore.
Therefore, the safest set-up is a short tether, on a tight jackline. If the jackline is too loose, it will sag far enough that you will go over the side. If you fall far enough that the tight jackline breaks a rib, just be glad you are not hanging off the side.
OTOH, like everything else in sailing, there are compromises to be made, and one must make their own choices.