To sum up? Is SN peaceful? Ehh, not really. It gets close. Compared to other methods that are this easily accessible, cheap, reliable and not gory? Yeah. SN is a pretty peaceful way to go. In the end it is still dying. There will always be some degree of pain or discomfort when passing away.
SN in my mind is just the least bad way to CTB that doesnt cost me a thousand dollars like Nembutal would.