You've asked similar questions before, the consensus is always no. I've talked to people who have taken SN, who have died and survived. No one suffocated, no one smothered that I personally know of that actually used it. SN is recommended in pph to begin with because it is peaceful relatively. No method is going to be without some discomfort. Gunshot would be faster, and perhaps therefore more peaceful. Full suspension hanging is faster than SN. Not many people want to do that though.
If someone can't get N or assisted suicide, SN is probably the best any of us can hope for and why so many use it.