There’s a lot to unpack when it comes to pistol braces, and I am by no means an expert.
Edit: there are threads on the legal subforum that may interest you, and some members here are actually lawyers (though not your lawyer as they might say), and you may be interested in reading their thoughts. Lots of videos online, too, just be mindful of who makes the video.
A short summary might be that some initial brace designs were ruled fine by the ATF, with the understanding that they were meant for people who are disabled. SB Tactical, which I believe stands for SIG Brace Tactical as SIG was the first to use them, was the original producer of the braces, though other companies showed up. SB Tactical started making other variants that could be argued to be much more like stocks than braces. The popularity of these “pistols” exploded from what they once were, with estimates of how many were bought being as high in some cases as millions.
Some would argue people were buying these as a way to get around the NFA, and over time the ATF took notice and made some comments, though stopped short of saying they were illegal (this lead to a period of time where some wouldn’t shoulder a brace, but instead prop it in their cheek because they thought the distinction would matter, some people got very nuanced). The ATF then came out with a ruling and a points system to determine whether a given brace and firearm combination was legal, as well as allowing for an amnesty period during which a braced firearm could be registered without needing the NFA tax stamp. That ruling was stayed in court, originally just for members of the plaintiff organizations that fought in court, with that being extended nationally afterwards. To my knowledge that is where it sits now.
Why this matters with the Scorpion is if you are buying say a 8” or shorter barrel variant, you are buying what is legally a pistol and have a few options. You can shoot it without a brace, with a brace, or go through the NFA process and make it a SBR. How you view all of that may impact how much the Scorpion appeals to you.