OK, the right reply this time.

Yes: soldiers fast-roping out of helicopters are indeed seriously vulnerable to fire from the ground. That’s why if they do that when they know there are people with guns on the ground, they will lay down heavy suppressing fire from the choppers or other methods of suppression. Also, it’s called “fast-roping” for a reason: you get your ass to the ground just as fast as possible, which means that if anyone is shooting at you, you try to make yourself a very difficult target to hit.

But a guy in a jetpack doesn’t have that option, he’s trying to keep the rig steady so he can line up his own shot. Without suppressing fire from other sources, the jetpack guy is boned.

Utterly worthless against an armed target, of course.

Try it against somebody standing stationary on the ground with a rifle while you try to line up a pistol shot in a jetpack that’s jittering around.