Last year I took a carbine course with a guy who used the 5.7x28 carbine. He was getting good accuracy out to 200 and 300 yards but the little rounds weren’t doing much against the reactive steel targets, even at close range. He complained that the trigger was spongy, though.
To me, ballistically, it’s not much more than a rimfire 22 magnum. I don’t see the big deal. You could be firing 5.56mm through a similar sized carbine.