Comment by imtringued

The reason why you want quasi direct drive with low gear reduction has to do with the kinetic energy stored in the gears. It's difficult to get an intuition as a human, because we don't have rapidly spinning disks inside our arms.

The moment your high gear reduction actuator is forming a contact with anything, it will have to either decelerate instantly, push the object away or move into the object by deforming it. If your robot arm is moving at speed and hitting a wall, the deceleration needs to shed all velocity in a few milliseconds. This is fine for the arm itself, but if you have a 100:1 gear reduction ratio, one gear is moving at 100 times the speed of the robot arm and since kinetic energy is 1/2mv^2, the energy stored in the gear is significant. Stop the arm and you'll break off the teeth on your gears!