To a certain extent it is, eg the introduction mentions that arrays should not be mutated but it is WIP.
I think it is best to understand that it is work in progress: don’t use it unless you are OK with encountering limitations (not necessarily documented), corner cases, issues, making an MWE, reporting an issue, and an occasional PR; and of course defining your own adjoints when necessary. As @baggepinnen, the issue tracker is very useful.
If you are not so adventurous, at the moment I would recommend Tracker, or even ReverseDiff or ForwardDiff, depending on your use case.