[Nexus 6P Q&A, Help & Troubleshooting] Funkwheat:
Agreed! I just went through your guide again, verbatim, this time with a TWRP data wipe beforehand and still no luck. This time everything flashed fine with the latest MTC20F images. Here's a command line transcript...

E:\angler-mtc20f-factory-d62de11e\angler-mtc20f>fastboot flash bootloader bootlo
ader-angler-angler-03.54.img
target reported max download size of 494927872 bytes
sending 'bootloader' (3526 KB)...
OKAY [ 0.105s]
writing 'bootloader'...
OKAY [ 0.209s]
finished. total time: 0.316s

E:\angler-mtc20f-factory-d62de11e\angler-mtc20f>fastboot reboot-bootloader
rebooting into bootloader...
OKAY [ 0.007s]
finished. total time: 0.008s

E:\angler-mtc20f-factory-d62de11e\angler-mtc20f>fastboot flash radio radio-angle
r-angler-03.61.img
target reported max download size of 494927872 bytes
sending 'radio' (48600 KB)...
OKAY [ 1.082s]
writing 'radio'...
OKAY [ 2.172s]
finished. total time: 3.255s

E:\angler-mtc20f-factory-d62de11e\angler-mtc20f>fastboot reboot-bootloader
rebooting into bootloader...
OKAY [ 0.013s]
finished. total time: 0.014s

E:\angler-mtc20f-factory-d62de11e\angler-mtc20f>fastboot flash boot boot.img
target reported max download size of 494927872 bytes
sending 'boot' (12201 KB)...
OKAY [ 0.302s]
writing 'boot'...
OKAY [ 0.182s]
finished. total time: 0.487s

E:\angler-mtc20f-factory-d62de11e\angler-mtc20f>fastboot erase cache
******** Did you mean to fastboot format this ext4 partition?
erasing 'cache'...
OKAY [ 0.030s]
finished. total time: 0.031s

E:\angler-mtc20f-factory-d62de11e\angler-mtc20f>fastboot flash cache cache.img
target reported max download size of 494927872 bytes
erasing 'cache'...
OKAY [ 0.023s]
sending 'cache' (5752 KB)...
OKAY [ 0.150s]
writing 'cache'...
OKAY [ 0.106s]
finished. total time: 0.282s

E:\angler-mtc20f-factory-d62de11e\angler-mtc20f>fastboot flash recovery recovery
.img
target reported max download size of 494927872 bytes
sending 'recovery' (13133 KB)...
OKAY [ 0.321s]
writing 'recovery'...
OKAY [ 0.195s]
finished. total time: 0.517s

E:\angler-mtc20f-factory-d62de11e\angler-mtc20f>fastboot flash system system.img

target reported max download size of 494927872 bytes
erasing 'system'...
OKAY [ 0.397s]
sending sparse 'system' 1/5 (478162 KB)...
OKAY [ 11.178s]
writing 'system' 1/5...
OKAY [ 6.861s]
sending sparse 'system' 2/5 (482961 KB)...
OKAY [ 11.220s]
writing 'system' 2/5...
OKAY [ 7.044s]
sending sparse 'system' 3/5 (480085 KB)...
OKAY [ 11.236s]
writing 'system' 3/5...
OKAY [ 7.268s]
sending sparse 'system' 4/5 (470078 KB)...
OKAY [ 10.822s]
writing 'system' 4/5...
OKAY [ 6.593s]
sending sparse 'system' 5/5 (55832 KB)...
OKAY [ 1.353s]
writing 'system' 5/5...
OKAY [ 0.808s]
finished. total time: 74.788s

E:\angler-mtc20f-factory-d62de11e\angler-mtc20f>fastboot flash vendor vendor.img

target reported max download size of 494927872 bytes
sending 'vendor' (185181 KB)...
OKAY [ 4.125s]
writing 'vendor'...
OKAY [ 2.974s]
finished. total time: 7.100s

E:\angler-mtc20f-factory-d62de11e\angler-mtc20f>

Btw, how long at the "Google" boot screen should I be waiting before I give up on it ever booting? I've been waiting about ten minutes. I have heard 20 elsewhere, but that sounds excessive.