I went from a chest harness in the "old days" to a seat harness and never tried a waist harness. I thought they would pull/stress my lower back too much where a seat harness wouldn't. I have always raced - long boards, course slalom and formula and the seat harness is the only way to go.
Here is one for you - I sail with my spreader bar hook facing up. In the "old days" with a chest harness, I would lean back so far that the line would slip out and I would fall back into the water. Solution - turn the hook up. Now, my lines only come out when I want them too. I tried to go back for a short time, but couldn't make the switch.
Occasionally, I accidentally get hooked in during a jibe, but that only happens if my booms are too low or if I don't bend my knees enough.
It works for me.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.