NixOS is a declarative distro. Meaning it you can declare pretty much every aspect of it from what software is installed to how the system is configured from a config file.
Using your calandar example, you can list Thunderbird (or whatever) as a package you want in the configuration and it will be installed. You can also use that same configuration on another machine and produce the same environment.
Relevant to the original point, since all your software is listed in a text file, you can easily see exactly what’s installed.
Risk is also a factor re: self hosting.
Those concerns are what stop me. Because I otherwise think I’d enjoy hosting a little corner in the fediverse.