I did a ton of mountain biking in the late 90s and early 2000s, after years of BMX racing, and one reason myself (and at least a few friends) stopped using bar-ends is because we kept injuring ourselves on them - they had to be constantly tightened or they would come loose while climbing or going downhill. I know more than one person that wrecked a wrist after a bar-end popped off and exposed the sharp metal edge of a handlebar.

I also biked with crazy people, so that was probably issue #1.