Back in 2016, a limited number of users began reporting a bootloop issue that wasn’t possible to fix by simply reflashing the device. As more users started reporting the problem, Google acknowledged the issue and said that a hardware problem causes the boot-loop error.
The good news is that Google is now replacing Nexus 6P handsets that are bricked and receive a brand new Google Pixel XL instead. If you think the deal is only valid for those with warranty, that is not the case. Google is even replacing the bricked phones that are out of warranty.