The JR Carbines tend to work their screws loose with a few mags worth of ammo, and I found mine would constantly FTF with hammer down on loaded chamber. Shame because it was a good shooter, very accurate with the Mossberg manufactured barrel. This was an updated version with the buffer fix.
The RFB has had several teething problems as evidenced by numerous posts and Youtube videos. It also seems to be plus-MOA accurate unless you shoot match handloads. Good enough for plinking and some short range (200-300m) hunting but not "serious" work. Others have had better success.
The XCR was a constant whipping boy with loosening screws, with matters exacerbated by the manufacturer's arrogance and lack of support. I seldom read about these problems now so either they've been worked out in latest production runs or users rectified them themselves.
NEA AR15s have a very sordid history involving rough machining, malfunctions, out of spec parts, spotty customer support, and of course the unethical shilling that occurred here several months ago. Some love them, some hate them, but it seems generally accepted that they are bottom tier in the AR world.
CSA rifles and carbines have been experiencing a fair number of issues. The latest post in the Red Rifles section is about an OOB ignition due to the barrel shifting 1/4" forward out of the trunnion. CZ858s made from ex-military parts seem to be more solid and reliable.
Not really a black rifle but the TNW Suomi M31s have been reported as having atrocious QC with barrel welds breaking and rough edges that need filing to make the gun feed.
As with any product, YMMV.