Comentário oficial
I've got a 2012 MBP (model 9,1) and to be honest, I've kept it back on 10.10.5, partly because when El Cap came out there were concerns, and then I got a new computer (2015 MBP), which I did upgrade from El Cap to Sierra. The new one works fine, but if I had to speak to upgrading, the users who have done an upgrade are the only ones who have every had networking issues (which was resolved with a fresh installation), but not all of them.