Guix has been utterly transformative for my scientific work. It lets me maintain a lab notebook (my shell history) with perfect description of what software versions commands I ran used. I don't upstream much. It's just packages I maintain in my own channel that support this. On a multi user HPC server environment, it's simply ideal. Every user can have their own system setup. Love it.
Similar to your comment the ability to see exactly what is installed, with repeatability is fantastic. For my personal use-case the ability to install an application in parallel to Ubuntu's is really useful.