I think this method has the potential to be reliable. I've seen several articles warning people about it and have heard of people dying accidentally from it.
I did have a practise in the bath last night, but realise I hadn't hyperventilated enough (or maybe I hadn't done it properly). I could hold my breath underwater for a longer time than usual afterwards (although I wasn't sure if I was supposed to exhale or inhale before I submerged), but never got anywhere near passing out. I've been practising hyperventilating since then, and can get a little more dizzy than I did last night, but can't make myself pass out afterwards.
I don't think the SWB method is 'unreliable', just because I can't make myself pass out, because I have read about people who can. If this was the case, then several methods would be 'unreliable', just because I haven't perfected the technique to use them. I can understand why to ctb is so difficult (after all, we don't want people dying by accident, just because they have held their breath etc), but it is frustrating having tried a few methods this month, but with no success with any of them. I think my next attempt may be with a plastic bag - I did start to try it previously, but discovered that my bags (including the backup ones) all had holes in them. Next time, I will try using a supermarket 'bag for life'. There's a satisfying sense of irony about it! Or maybe I'll just die in my sleep tonight as a result of all of my hyperventilating attempts - that would be even better!