The Mac minis make adequate, but very low spec servers. From what I understand, their disc IO sucks (because they use some old laptop drives or something). Moreover, I've used a mac mini as a server hosted by Mythic Beasts- it's not that fast.
Moreover, there are some hardware features which make them rather unsuitable as servers. Some models won't remember their power state correctly, hence remaining off if power is lost and restored.
I'm talking here about running Linux on them rather than MacOS.
Mark