1) internal HDD can be replaced with cheap, big SSD
2) Computer is absolutely silent
3) everything in one package, no mess of cables
4) very reliable
5) very low power usage
MacOS isn't the best OS for running a server, but I'm used to it so I don't mind setting up launch agents instead of systemd units
Much better value and power over a raspi.
- https://www.servethehome.com/introducing-project-tinyminimic...
If you put them into a cupboard, those PSUs take almost as much space as the computer itself.
That being said, if you use the Mac as a server, behind a NAT/Firewall with only some ports open, use up to date server software, and don't use it to surf the web, then the security impact of using an outdated OS is minimal.
You could of course also just install Linux on it.