Citing various incompatibilities and hardware deficiencies, they cut out a large swathe of machines from running Sierra. With macOS Sierra, Apple dropped support for some of its hardware models for the first time in several years.
I did the Sierra upgrade on a couple of MacPros 4,1 and it has worked excellently, after I did a firmware update patch to get it to read as a MacPro 5,1, so I am very optimistic on doing it again. I am currently researching installing OS X 10.13 High Sierra on unsupported Macs. * The “macOS High Sierra Patcher” application is intended for those with unsupported Macs and Hackintosh users, but any Mac user can use the app to be able to download the complete installer file from Apple servers. Select the location and it will start downloading. It will ask you to select a location to save the macOS Sierra Installer. Here just go to Tools-Download macOS High Sierra as shown in the below image. It will again open the macOS High Sierra Patcher. Next, select your USB drive in the Target Volume list, and click 'Start Operation.’ When the operation completes, boot your target unsupported Mac off the USB drive you just created. Open the ‘macOS Sierra Patcher’ tool, and browse for your copy of the macOS Sierra Installer App.Ensure that the tool successfully verifies the app.