Curious how folks feel about an alternative not mentioned here: Shepherd. It's what guix uses. As someone a bit singed from trying to get Nix to a happy state I'm attracted to guix's more straightforward design and Scheme-based compiled configuration. On paper I'm also impressed with Shepherd's design too, but switching out such a sprawling core piece of modern Linux leads me to anticipate future pains from various software that assumes I do have it.
