Since 2015 I don't think they have been any good. 2012-2015 are the best ones imo. 

Careful with the mac minis, they overheat a lot. I wouldn't count on them lasting particularly long. I'd just get a normal desktop and install your favorite distro.

@Bananasplit_00  I think that 2016-2018 machines are just a medium to something more profound.

but at this stage I don't have high hopes for apple. have you heard the rumor that since 2016+ machines got keyboard problem apple might add in another layer of glass between the switches and the whatever it is down there? can you imagine how much will it cost if you want to repair it?? I think mac mini is so much more robust and functional. I am still really worried about my 2018 machine got problem after 1 year warranty expire. might have to purchase applecare. 

@Sauron have you got concrete evidence that 2018 machines got over heating issues? I know it is apple. So it will thermal throttle. But I think the longevity of the machine will not be compromised. I have at home a 2012 mac mini that works like a charm. I still WANT at least ONE mac in my home. I prefer it over linux.

I will probably get the 6 core i7 machine + 8GB of ram + 256 GB of internal storage for my mac mini and use it as long as possible. later add in more ram.

I do have evidence https://appleinsider.com/articles/18/11/14/testing-thermal-throttling-and-performance-in-the-2018-i7-mac-mini

It reaches 100C and very quickly at that, which is as high as an Intel chip will go before thermal shutdown to prevent instant damage. It literally cannot overheat more than this. This has obvious consequences on the performance, but more importantly on the longevity of the system - as soon as any dust gets in the ventilation system the throttling will become increasingly more significant; furthermore, while the CPU probably won't die from heat, other parts might very well die sooner rather than later.