My daughter got a ground work kit for her birthday and her instructor explained to her friend's parents (a party at the barn) that longing/ground work can help when a horse has a problem under saddle, but it gets the rider off.
Yesterday we were invited up to a trail clinic at the farm. In the covered arena there were deer and coyote decoys, tents, tarps, balls, umbrellas, ribbon, etc. The first few hours of the day were the riders longing their horses near all these obstacles, over the tarp, etc. Every last horse had an issue with the tent or coyote, but after spending time longing near them, poking at them, etc, after lunch all the riders were able to ride the trail course without being thrown. 3 of these horses are known for being hot, stubborn, and flighty. They benefitted most from the ground work before the ride.