Radian owner here. So the LGS owner is technically wrong but he does bring up a relevant point. For starters, I am not aware if any malfunctions that can’t be cleared on a Radian lower. Double feeds are super easy.
That being said, a Radian lower can possibly make the extremely rare brass over bolt or bolt override malfunction a little more difficult to clear. The LGS owner was probably trying to say that there are certain positions of the BCG’s path that will prevent upward movement of the bolt catch which may lock up the mag release.
On my lower, if I induce a bolt override malfunction, the right side mag release gets completely locked up but for some strange reason, the left side release is able to move just enough to allow me to pull out the magazine with a gentle tug. This isn’t really an issue though because even with a bolt override, you can usually pull the charging handle back just enough to lock the bolt back and at that point, both mag releases will drop the magazine.
I wouldn’t worry about it honestly. The bolt override malfunction is extremely rare. I’ve never once seen it or experienced it and I remember Jerry Miculek saying he’s only seen it twice in all his years of AR shooting. Worst case scenario, if it does happen, you can probably still get the magazine out and clear the malfunction.
Hope this helps.