Have you read the PP handbook? It discusses reliability in depth in there.
I saw one post on here that linked to a scientific article where someone took N with a load of other stuff, but she only took about 3.5g I think? And she was saved with no ill effects because she was discovered and treated, with such a low dose I'm not surprised it failed.
The handbook talks about how some people due to their medications, or chronic alcohol abuse, have built up a tolerance to barbiturates and as such went into a coma for 24 hours before passing, but I think it says nobody has reported to them a straight out failure with the full 6g dose. That's why they recommend making sure you're not disturbed for a good while just to make sure.
Can anyone corroborate if I am talking out of my ass or not?