Super cool, interesting, and highly innovative. With that being said, I'd be nervous as hell to use one at a range that wasn't 100% private and far, far away from everyone in fear of being hassled by people, especially including those with badges. Using one in a registered SBR may help deflect some trouble, as you have "paperwork" for such an exotic weapon that may ward off problem people to request a peek.
I'm fairly certain the ATF wouldn't let this fly, if they were to evaluate it at their no-fun-for-you facility.
As far as it being legal, it appears so under a reasonable interpretation of the law. But unfortunately, that doesn't seem to matter these days. (Bumpstocks)