Lo and behold...it just happened to my own Nexus 5X tonight. I had been having some unexpected issues (including some problems accessing the phone from a PC over USB) but didn't realize what was up. Tonight, it suddenly locked up while browsing through the UI and it would not boot after that. I popped it open and briefly disconnect the battery. After that it would power up again, but never complete a boot cycle. Fortunately, this also meant it could be booted to Recovery Mode.
After some wrangling (which included freezing the phone - literally, as in get the unit down to abou 40F) to briefly get around the boot loop so I could access Developer Options and enable OEM Unlocking, and also installing ADB+Fastboot on my PC, I was able to flash 4CoreOreo.img onto my v8.0.0 phone and reboot. One factory reset later and it looks like we're sort of back in business, although it's still prone to lockups or spontaneous reboots if it gets warm, and it definitely hitches a lot more than usual.
Not ideal but at least I can get into the phone.
EDIT: I ordered a new Pixel, case, and screen protector on Amazon and paid through the nose for 2-day shipping...should have it before the Thanksgiving holiday travels. No way can I rely on this 5X to coordinate with my wife through airports. Goodbye, $600