Yeah it's pretty "terrible".
But here's the thing, out of the methods we can name comparatively name that people have ctb with out there, SN is just way too nuanced to say it's a "terrible" way to go as we've seen here, discussed on numerous occasion whether an attempt was close to working and the user described it as peaceful, relatively so or described as a terrible experience where one particular symptom was too distressing to deal and eventually led to them calling for help.
What's a guaranteed death anyway? Nothing out there is guaranteed to work, that's just part of the risk aspect when it comes to ctb so I don't know how this is a problem of SN solely and as for its popularity, I think it's been explained multiple times already as to why people either choose it as their main method because they've ran out of options that would've been a better alternative or keep it as a backup just in case whatever plans they have to get their main method don't work out.
We've seen times varying degree to what extent all the symptoms have affected users who've survived and I still don't find them particularly distressing, and even they've described as so. The success of this method, just like any other, depends on the level of preparation you're willing to put in, whether it's taking responsibility for your own research and getting the correct items needed for it to work but statistically, I think there's already been a few medical articles out there that give an idea of how the response to this method has been, it's lethality is characterized by Methemoglobinemia which can induce tissue hypoxia and may then lead to cardiovascular collapse, causing death by acute respiratory failure.
I've seen opinions readjusted around this method more than I've seen for any other that's led to misinformation and overall, a sense of fear, at this point, listen to the facts, discuss accurate sources and take it from there.
I think anyone can say sn is a terrible death if it doesn't fit the definition of what a peaceful death is for them and what a terrible one is, everyone is different and I can respect that but there's just too much information out there right now to say it's an excruciating, agonizing death or saying that's it's completely peaceful without any discomfort. We know better now than 2019 so there's just very left to discuss on this method that hasn't been.