Better than nothing.
You’ll shoot your eye out with one of those. As an aside, one of my neighbors was blinded in one eye when the surgical tubing busted when he was pulling it back and it hit him in the eye.
Wow, it's been at least 40 years since I last heard that product mentioned and probably 50 years since I last used one and even then I couldn't hit anything. And it's probably classified as a weapon anyway so you might as well carry something decisive if you're leaning that direction. Better to be tried by 12.... But if you're good enough to get close enough with a sling shot and can really put the ball bearing on a lethal piece of skin while under an active attack then I submit there are better options than a wrist rocket sling shot.
Check your local laws first.