It's hard for me to put my finger on it exactly but at this point NUCs just feel more hackable. For me, that's why the mini is not a satisfactory replacement.

If you want a little kube node, if you prefer the way ubuntu and debian upgrade over the way osx upgrades (and have the linux experience be decent), if you want to shuck it and slap the board in a robot and run ROS, if you want it to be a steam box with an eGPU, there's a bunch of things NUCs do well / places they fit that mac minis don't quite (yet, at least).

Mac minis are great but they're not the same thing. The "tight hand" of Apple makes them great but also limits them. Like you probably can do all the above, with sufficient effort but is not a first class experience, in the same way that an orange pi is not a raspberry pi.

The last mac minis i worked with were the final gen intel ones. Very similar to a NUC, you could run whatever linux on them fine (and even windows with the bootcamp drivers). I think we'll get to m* series mac minis running linux eventually, it's just going to take some time to get it all ironed out.