The flash on every phone out there is dogpoo...
I'm coming from my experience with my Nexus 6 (no 6P... yet). And while the two cameras are totally different, the HDR+ does the same thing. Between standard and HDR+ mode, the HDR+ picture almost always comes out better looking... provided things like movement in the frame don't screw things up. HDR+ basically takes several shots and digitally processes the RAW file to kind of smooth out the lows and highs. You may lose a smidge of the finest details, but the resulting image is usally better composed with a lot less noise.
Since GC 2.5 dropped when the M preview launched, I've been leaving the phone on HDR+. And with GC 3, switching modes is a LOT easier and orders of magnitude faster. So if HDR+ ain't working out, you can switch to normal or aHDR.
as for Auto HDR+.... playing around with it, it really only seems to trigger HDR+ when it obviously needs it. But I've found that even brightly lit pictures benefit from HDR+ (and they as fast as normal in bright light).