I'm using OSX 10.11 what should I be aware of when installing 10.12
Going from one major revision to the next, there could be a lot of things if you are a developer (which I am not).

The smart thing to do is to make a full bootable backup of your boot drive (CCC and Super Duper to name a couple I am familiar with). Then TEST the backup by booting from it. Then unplug it from the Mac and put the backup drive in another room.

Do your upgrade and then check your environment, your builds, your tools and make sure everything is working as expected.

Now if things get messed up all you have to do is boot from your backup drive, wipe the internal (boot) drive. Remembering to save any changes to files you made in the interim to your backup drive. And clone the backup back to your original boot drive.

With any luck you'll only kill a half day or so...