^^^

i've got PB/PDW brackets on my rear rack, and under the saddle with a slightly modified water-bottle holder. both give a pretty stiff ride to whatever lights are mounted there, but i suspect the rear rack would be stiffer. both times i had a real-world turbo failure they where on the saddle mount. i've never had this type of failure with a PDW light.

mounted on something softer (jacket, pannier, etc) i suspect this wouldn't be a problem, but then aiming the light properly is pretty much impossible.