"If you want a powerful server that you can stick in your rucksack or airline carry-on bag, there's not much to match this currently."
It really depends on your use case, but I think there is. For one thing, you have to figure out why you need a powerful server on the go. If it is to deploy it somewhere else, then you still end up in the small computer comparison. I'll grant you that sometimes you need six cores and 64GB memory on a server that you carry with you, but most of the time, probably not. The ease in modifying a NUC makes it a really good candidate for to-be-deployed server in a small box.
In most cases, however, the computer you carry in a rucksack to provide you with a lot of power will be a laptop. You can get them with more processing and memory, although those are usually somewhat heavy. I assume that would be a more useful portable machine than a mac mini for most use cases. You can compute on it when you're on the go, rather than having to have mains power, a monitor, and input devices to run it.
The mac mini also has lost some of its appeal when compared to other options in the small computers lineup. It is far too expensive now to be a logical choice for a media machine, especially as a raspberry pi can do it well. For server purposes, you would probably want a device running Linux on more expandable hardware, rather than using the OSX server app that has been losing functionality.
As a mac desktop, it's not bad. It has the ports that we normal people want, and it is reasonably powerful. Still, it has lost some of the niches a mac mini used to fill.