SN does seem to be moderately unpleasant. Aside from reliability, the generally accepted protocol aims to mitigate as much of this as can reasonably be done. SN just so happens to be affordable and more accessible to a lot of people than more pleasant methods are anymore. The failure reports honestly terrify us although it seems most are avoidable by adhering to protocol strictly.
Inert gas would be the closest to N as far as how pleasant it would be, and even then, SI can kick and cause one to abort an attempt. Inert gas just happens to induce unconsciousness a lot faster than SN does and doesn't have all of the unpleasant physical effects to deal with and doesn't usually trigger a panic SI response.
CO, done correctly could be quite peaceful. So many people unfortunately unintentionally pass via CO poisoning every year without even knowing they were exposed to it.
Since we see the cardiac switch method mentioned, we doubt that is really pleasant either. Sedatives can help deal with this while sedatives can potentially ruin a CTB attempt with SN as one may lose portions of their dose, even while unconscious with no way to follow it up with a re-dose.
Unfortunately, no method is perfect. The human body is quite resilient and choosing to CTB means fighting an entire evolutionary history that made us more resilient. After all, humans can safely consume many substances that are fatal to many other species with minimal or even no side effects from them.
Methods that restrict blood flow to the brain such as partial, NN, and full suspension also don't seem very pleasant before one hopefully loses consciousness. And there seems to be a lot of variance. For instance, we have tried many times to find our 'sweet-spot' but can't induce loss of consciousness without also triggering at least some panic. Even when we are clearly cutting off blood flow to, rather than from, we just can't get to the point of unconsciousness in a reasonably safe trial.
It really is a tragedy that N may as well not exist anymore for most. It seems that the only unpleasant aspect of N is the awful bitter taste. Even with such a near ideal method, some can't stand the taste for long enough to consume it.