Never trust an Apple upgrade.
I didn't upgrade to 10.12, but a couple days ago that became necessary as I needed to install a later version of XCode. But 10.12 was no longer available (anywhere) so I took a punt on 10.13. This of course broke my VPN and Elixir/Erlang dev env, so I checked out the options for rolling back to Sierra. None. The only option was to obtain another Mac and use the High Sierra mac as a table mat.