I have an old mac mini that was a server for a good 4-ish years.

The good:

1. They are pretty good at sticking in a closet and forgetting about them.
2. Specs are always on the decent side and some of the older models are easy to upgrade.
3. Power is ok. It sips power
4. It can run for years without issues. I still have two mac minis I used for CI/CD jobs, thin clients, etc...
5. Its a cheap mac. If you need mac for something, like building custom mac specific applications, then its a decent little machine.

The bad:

1. CPU is usually lacking compared to any computer of the same price range.
2. MAC OS. Its good at desktop but as a server, it just doesn't have the same options/ease of use as a good linux box. You can get around that by dual booting, but its just another headache. Docker/VMs are also an option, but the RAM/CPU usage would take a hit.
3. The newer the model, the harder it is to upgrade.

I would use it as a specialty server if you have something you do automatically only macs can do. Or as a thin client/vm box.

I used to use it as a CI/CD box before github actions was a thing. If you happen to have one, sure set it up for fun. If you dont and are looking at buying one, I would suggest a cheap dell desktop or (depending on what you want to host) a pi 5 or thin client and throw linux on it.