Its about the length of time, the torso weight on the arms and hands, the angle of the wrist to the arm.
Time: I think you already know this. Practically speaking, a good practice is to come to a stop and rest the body.
Torso: in a road bike position, a rider is leaning forward and the tendency is to apply the weight onto the arms and thus the wrists. Some will argue that a rider needs to develop the back & front muscle strength by doing "such and such". Also losing body weight. LOL.
Wrist to arm: This is where I found some relief. When I hold the drop bars, the wrist may be bent or may be straight with the hand. I found the wrist in a straight alignment with the arm and hand is better in that it may not cut-off circulation or nerves. This is purely my conjecture/assertion. Just a thought.
On drop bars, its possible to align the hand/arm this way.