Ultimately everyone's individual biochemistry will respond to things differently.
What some people might describe as "unbearable," others might be able to easily shrug off. In the same way all of the reasons that bring someone to this site are personal in the pain they give to the individual.
Not to mention, most people when they are attempting are going to be in a heightened state of emotions or anxiety, which will amplify every little discomfort and SI can bring about a state of uncertainty or regret toward your decision in the moment. Those who report back on the effects are naturally only the ones who failed for one reason or another. (There have been cases of third parties who witnessed someone CTB by SN report on their subjective judgement of whether they felt it was painful or not, but they were not the ones who took SN.) Those who were not so bothered by the effects would be less likely to ask for help (assuming they were not interrupted against their will and that they were able to take the entire amount) And there is also the consideration of following the SN regime in its entirety, which will also help it be more peaceful, that fewer people choose to do.
Based on what I have seen, I consider it to be a peaceful method in comparison to the alternatives, though others have come to a different conclusion.
I feel many users make a mistake in trying to hold every method to an near unattainable gold standard of being 100% peaceful, with 0 discomfort, 0 permanent or even temporary damage if failed, on top of being easy to access and prepare for. Pretty much nothing will check all of those boxes, not even N (being that the price and limited vendors are primary barriers to that method).
I think if someone is too scared of a method itself, SI will undoubtedly impede them even more and make the experience more painful, psychologically or otherwise, then it should be. In that case, one should either find some way to assuage their fears, or find another method.
I am willing to experience the effects that I have seen most commonly in SN CTB. Others might not be. That's a choice you have to make. Just ensure you can gather as much factual information as you can.