It's a matter of correcting torque on the front-wheel. One component comes from the amount of tail due to the front-end geometry. If you hit a bump and the wheel gets tweaked by say... 5-degrees, that generates a lateral force on the tyre pushing it back into line with the contact-patch. The torque on the steerer-tube is then Force*Trail. The longer the trail, the more torque on the steerer-tube.

Up at the handlebars, it's a similar thing with the hands pushing on the bars to spin the stem. You have a given amount of push on the bars to spin the steerer-tube. The further your hands are away from the pivot, the more torque you'll have for a given amount of hand-force. With longer stem + longer reach bars, your hands will be farther away from the pivot and will exert more torque.

So... when you hit a bump, a longer-stem results in less force at the handlebars for your hands to resist. And conversely, it takes less force at your hands to counteract bump forces twisting your front-wheel. I prefer the feel of a tight, fast-steering front-end geometry (74-degree head-tube, 45-50mm trail) along with a longer stem to keep it all stable.

Another thing to consider is how much your hands have to move in order to generate a given amount of turning. With shorter stems, your hands have to move a smaller distance to generate the same angular change say... 1 or 2- degrees. With a longer stem, the same amount of hand-movement generates a smaller angular change and less response from the bike.

You also have to add enough data-points and experience to differentiate. I raced for 10-years straight from 1988-1998 and had roughly 12-bikes during that period (6-years of track racing on 2 different bikes as well). Did over 400 races during that time (not counting TT or TTT). On some of these, I've tried 4-5 different stem & bar combos to fine-tune the fit and handling feel. It's not an issue of "good" or "bad", but rather of feel and control.