Unfortunately, I can't really say whether or not runsit is an interesting entry in the group of process management tools. While I'm very willing to dive into the code if I run into issues or am looking to customize things, the lack of any documentation or overview has prevented me from trying out runsit yet. Partially, this is for the obvious reason: it's difficult to get started with a tool that doesn't document how to use it. Additionally, the lack of documentation is a warning sign that this project won't be maintained in the long term as a community project. Which isn't to say that projects that begin without a strong focus on documentation or community cannot change in the future, but in my experience the vast majority of these projects end up being "a thing the author wrote for themselves, which becomes abandoned when they no longer use it".

As far as the variety of other tools:

launchd is an interesting concept but hasn't picked up a ton of adoption by things other than OSX.

I've used supervisord for some other projects, but never as the main init system, and the python dependency has thus-far prevented me from doing so: I like my init process to be statically compiled so that the initial system startup has as few deps as possible.

I really enjoyed runit; I'm only trying out s6 now because it has a few more helper tools for controlling process startup and management, and appears to have better support for log handling. Runit was very powerful as an ultra-light-weight init system, and both runit and s6 have very thorough docs, with runit having a large catalog of community-provided initscript examples.