After trying on the Nexus 6P with HDR+ always on yesterday, I wanted to evaluate HDR+ Auto in typical situations where you want multiple exposures to recover shadows or highlights.
As you can see from the samples shared here the result is clear: HDR+ Auto simply doesn't work.
In all my attempts today, HDR+ Auto was unable to identify scenes with dynamic range challenges and in need for some tone mapping.
I remember that yesterday HDR+ Auto turned HDR+ On at least once but I was unable to reproduce that today in real world scenarios.
– The 6P camera exposure system can easily underexposue a central subject. It is common that you need to aid the exposure system by tapping on your subject (hopefully your subject won't be of dark color)
– HDR+ sometimes increases contrast and reduces the final dynamic range instead of extending it, giving the opposite result to what's desired. It does so unpredictably.
My tip would be to activate HDR+ as forced On since HDR+ Auto doesn't activate it in the obvious conditions requiring it on, unless you need to shoot several images quickly.
Then since the automatic exposure can't be trusted outdoor, you it is recommended to tap on subjects to expose for them, while the preview will often seem overexposed, HDR+ should usually re-expose the final image rendered and recover highlights in the process.
HDR+ in general needs work to avoid being counter productive randomly, and HDR+ Auto is useless as it is now.