koala logs

Raising your left hand in such manner might make motorists think you're signaling to turn right when you might actually be intending to turn left

Also hitting bumps while one hand is off the handlebar might cause you to crash. You won't be seeing the incoming bumps either because you're focused on the mirror. At least if both hands are on the handlebar, even if the bumps surprised you, you won't crash. I still vote for helmet mirror. Glove mirror is potentially unsafe unless you're dealing with perfect roads all the time with very few cars on the road to confuse with unintended signals.