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.