It has all the benefits associated with the ideal overdose (peacefulness, simplicity of use, gradual drift away, fewer or no real drawbacks from failure, etc.) without the main drawback (it's highly reliable, unlike most overdoses). It's particularly popular because it's used by end-of-life clinics and endorsed by organisations like Exit International, which gives you some insight onto the considerations they have (their primary audience is terminally ill or end-of-life planning older people, who may not be able to go about other methods or who rate overdosing as more dignified than most other methods, but otherwise can access the stuff they need if they plan ahead).
Its primary drawbacks are that it's hard to obtain since it's now tightly regulated in most countries (drugs that are less lethal when used in suicide are perceived as safer and replaced it on the market, so these ones were phased out) and accordingly it's expensive compared to most methods (and for some people, having to import it is a dealbreaker). It also tastes rather bitter.
The Peaceful Pill eHandbook has a lot of good starter information about Nembutal, you can read it in the Resources thread https://sanctioned-suicide.net/threads/list-of-resources.3/