As a (washed up) climber, falling is something I think about a lot. It turns out that sit harness used by recreational climbers are unsafe. In a catastrophic fall, the harness directs force into the lumbar spine in an anterior direction. An old study on industrial fall arrest found that loadings in this orientation can produce spinal injury around 4kN (don't quote me on this number, it's been a year since I read the paper.) It gets worse. If a falling climber rotates during the fall, load will be applied left-right above the pelvis. This orientation decreases the spinal injury threshold further.
A chest harness, on the other hand, loads the body in a vertical orientation applying compressive force to the spine. This pushes the survivability margin out toward 7kN and beyond the maximal loading forces you can experience in a factor 2 fall.