MAC is spot on, here and worth a watch. I would agree on the recommendation - if you want to play it safe with an AR-15 pistol, get one that comes as a pistol from the factory. Sig's pistols, BCM's pistol or similar from other manufacturers.
Then, it's sold and transferred to you as a pistol, says it's a pistol on the box, marketed as such, and the manufacturer's intent is without a doubt - it is clearly a pistol. This prevents any kind of confusion and potential legal entanglements about 'intent' to manufacture an SBR that could come up with a DIY build.
That said, the ATF hasn't come forward to clear up matters at this point. They could clarify that shouldering an SB-15 brace occasionally, even on a factory made pistol, is a no-no. At this point, I wouldn't be surprised if that was the final outcome. Good luck on enforcing that one, though.
If such an opinion is released, I would expect it to apply to the various SB-15 knock offs like the Thorsden, the Sig SBX brace, etc.
Personally, my interest in an AR pistol has waned. I've decided against 300 Blackout - logistics being the deal breaker - which points me to a 10.5-11 inch pistol in 5.56. With muzzle devices installed, such a pistol offers only 3.5 to 4 inches of barrel length savings over a 14.5 inch rifle with a pinned comp...and the pinned 14.5er is legally a rifle, versus a gimped pistol and the various associated downsides.
Any thoughts from the tribe? Tired of this mess yet?