I don't think this is a big secret. If I recall correctly, he taught a class in a shoot house, posted a video of it on the internet, and got jumped on, hard, by some knowledgeable people for some potentially messy safety issues. I never saw the video and am not qualified to say what was or was not done correctly. There was a big thread on M4C about it, which may have (has probably) disappeared by now.
I've bought stuff from Grant, usually when he has specific gear in stock that other vendors do not. His prices are sometimes better as well. No problems with service on my end.
I referred him a friend who wanted a custom AR upper (i.e., an upper with a specific rail pre-installed on it). He never returned my friend's emails or phone calls.
YMMV.