I think SN overall has low physical risks and a high chance of success if done correctly -- fast for 8 hours, take the right amount of SN with the right amount of water, and don't get interrupted. If one wants to abort, they can and be able to experience a full physical recovery, but may face a psych hold after treatment. It's not a comfortable method, it depends on what one can tolerate and how their body reacts, but for most it's discomfort for an hour or less.
I'd recommend reading the SN Resource page and Stan's Guide to SN, both are in the pinned Resource Compilation, as well as the thread that's gathered all the anecdotal accounts of SN successes and failures, and the PPeH if you can access a copy. There are screenshots of the PPeH SN pages included in comments in various SN threads, I think in the past month or two they've been included in the SN megathread.
I definitely recommend looking through the Resource Compilation, I might have overlooked something, and it's a great resource.