The Mac mini represents an enormous compromise. It's rarely, if ever, going to be the best form factor for any given role. Hell, it doesn't even have "cheap" going for it this time around, really: the 2014 version at launch had a base price of $499. The hike to $799 represents astonishingly bad value. Though spec-wise the new machine is unambiguously superior to the old one, it's by no means a high-end machine. It simply scratches one crucial itch: you're wedded somehow to the macOS ecosystem and need a computer.
That build farm? The Mac mini is a crappy choice for a build farm unless you really need to build software for macOS and iOS systems, in which case, hey, it's essentially the only option Apple has. Need a Mac because you're wanting to explore iOS development? Get the Mac mini and continue to use the keyboard/mouse/display that you've already got on your desk. It's no longer the affordable way into the macOS environment that it once was, but it's the best thing Apple really has to offer.
This is, I think, the general theme of the mini. I struggle to imagine any scenario where the thing truly shines—but Apple's refusal to build, y'know, a regular desktop computer means that the mini often ends up being the best option if you really are committed to using macOS.