I have a 2017 27in retina iMac booting off of a (stable) DIY m.2 usb drive (controller: Realtek RTL9210 NVME). I upgraded to Monterey (public release - NOT beta) on Oct 25, the day it was released. It seems to have installed macOS 12.0.1. It upgraded and booted fine, and I interacted and configured the system (I changed app settings that were new with Monterey). When I shut it down (night) and booted it up (morning), the boot progress bar wouldn't cross 1/2 (everything else was normal). I tried booting in safe mode, but got no luck. I reinstalled Monterey with recovery mode (DID NOT WIPE DISK). However, I had the same issue. I tried safe mode again, no luck. I reset NVRAM and tried again, safe and non-safe (no luck). I booted into verbose mode to get the output, and it didn't seem like something I could fix. Here is the output:
Finally, I tried booting on a 2015 iMac 5K. No luck there either.
I needed my Mac, so I reinstalled Big Sur (fresh) from an external boot drive and used a TM backup to restore my data. Unfortunately, this means I cannot perform any more tests.
My main questions are, what went wrong (curious), and if/when I can install Monterey again (I would highly prefer upgrading, even if I have to wait a month or 2). I would prefer not to, but am fine if I need to re-install Big Sur, but I would not like to right now, so I will most likely execute suggestions in a month if there is not a high level of certainty.
P.S: the only kernel extension I have that I know of is VirtualBox
Thanks again to all who answer my question.
Edit: see the solution I posted