I see opioids as a better alternative, but there are a few things to consider and I think those are the reasons why opioid o/d have a high failure statistic:
- Street vs pharmaceutical: differences in potency and quality. One is most of the times cut, the other is not.
- Tolerance: do I have to state the obvious ?
- Dose: again, the tolerance has to be taken into account and a fatal dose for some could be another day at the office for others.
- Mixing or not mixing: for example alcohol x1,5 the potency and increases the chances of respiratory depression. Some will take the drug alone and fail, some will take the same stuff or the same dose and o/d because (or thanks to) the added "ingredients", such as alcohol, benzos or any famous CNS depressants.
- What kind of opioid ? : there are plenty, but not all work the same, they don't work the same on everyone and not all of them are good to o/d on.
- The route: opioids taken orally are known to make you vomit when the dose is too high; for example transdermal patches or IV are a whole different game.
- The timeframe and being alone or not: it happens a lot of times that the subject is found; not only because he/she had a friend passing by or family they lived with, but because dying from o/d takes quite a time; and since Narcan is very good at what it does, here you go again.
So yes, opioids could statistically have a better success rate if the subject had no tolerance, a large enough or potent enough pharmaceutical grade opioid taken via transdermal or IV, access to other substances that potentiate it and zero chance of being found until it's too late (like >15 hours minimum). And I think those are the main reasons why a lot of people fail with this method, it's not easy to check all those boxes and let's be pragmatic about one thing: a large part of those who try this method had access to the drug, and chances are if they had access to it in the first place they were using and their tolerance was high enough to screw it.
SN, given how it works, is also very powerful stuff. I'm definitely not a fan of the substance and would never CTB with this because the side effects scare me, but I understand it's "success" since it's simple, straightforward and deadly af.