The only issue I’ve had is that Xcode 7+ tends to crash frequently while editing – it has something to do with the autocomplete or lookup database in Xcode… so I tend to save more frequently.
Do you have any widely deployed products that you built + shipped a new version of, after upgrading to a new xcode?? i.e. no issues with end users there?
Xcode 8 (I’m using 8.2.1) on Sierra can compile JUCE applications and plug-ins which are compatible with OS X 10.6 (and possibly older than this with a few tweaks).
If possible I would recommend using the latest version of Xcode with JUCE.