Still using multiple Mac Mini 10.12.6 servers as AFP, CalDAV, Web (http links for clients), Open Directory and VPN services for our company. Works great. Especially for remote clients who can't log in by using SMB has multiple ISP's block that port.
Does anyone know the specific reason why the author mentions "4,1 Mac Pro with the firmware flashed to 5,1". I looked around on the web and don't see any issues with people who flashed a 4,1 to 5,1 and then upgraded to High Sierra.