While this saga of continuous reboots began with the LG G4 and V10, it had spread to the Nexus 5X in mid-2016. All those phones have the Snapdragon 808 chipset. But even the Huawei-made 6P, with a Snapdragon 810 running the show, still fell to this syndrome.
A few industrious minds at XDA-Developers do have an idea, though, and have figured out a fix to the situation: by deactivating the BIG cores on the chipset. Yep, apparently turning off that the set of Cortex-A57 cores and letting the lighter-weight Cortex-A53 run, you’ll see performance drop off relative to normal, of course, but at least you can get past the Android bootscreen. We don’t know exactly why this is the case, but the fix has gone through multiple users’ phones very well.
An image and TWRP build based off the July security patch are available from the source link along with instructions on how to execute the installation. If you have a Nexus 6P dead in the water, a little know-how might just be able to revive it.