I have a 2018 Mac mini with the 27 inch LG UltraFine 5K display, which for me is a lovely combination. However I also have an old (mid 2011) 27 inch iMac, so why not use that as a secondary display?
Because once you’ve used a high DPI display, you will never want to go back to using a “low resolution” display like that on the iMac. But I still want a second monitor!
Luckily, the 2018 Mac mini only supports one 5k display, otherwise I would have been tempted to buy a second LG 5K. It will, however, support a second display of up to 4K connected via HDMI, so I bought a (much cheaper) 27 inch LG 4K display with HDMI input to use as my secondary display, and I haven’t looked back.
I did have some success in using the USB C Luna Display to extend the Mac mini’s desktop to the iMac. This isn’t a supported configuration for the Luna Display, because it officially matches the Apple technical specifications, but if you extend the desktop to the iMac and then unplug and reconnect the LG, it will display over both monitors. The problem with this hack is that you have to do it each time you reconnect the monitor (e.g., after a reboot), so it’s a bit pfaffy. Also, it turns out, the iMac gets hot, even when just used as a display.