They're a novelty, at best. Only if you have the money for the ammo. Those things will chew through your wallet. Can easily blow hundreds of bucks worth of ammo at the range if using factory loads. Besides, they're really easy to jam up because most people don't know how to properly shoot a rifle like an AR.
Yeah you can buy a legit one if you live in a state where it's legal, plus you wanna spend $20-30k on a legit Class III machinegun. Go through the proper background and pay for all of it. As the old saying goes, ATF should be a convenience store, not a government agency. Gotta have money, land to use it in, not live in a commie shithole state.