I have yet to find a phone whose auto-brightness didn't annoy the crap outta me. Unfortunately the nexus 6p is no exception in that regard. I tend to keep auto-brightness off and set the display to ~50%. I'll occasionally have to dim it more in really low light or crank it up when in direct sunlight.
It seems on every phone I've had auto-brightness is way too conservative for my tastes in low or moderate light, so the occasional manual adjustment (especially given how easy it is to get to via the settings pull-down) is worth the effort.