An update here, I have been able to reproduce the error case. It does not brick the unit, but the final piece of the
particle update fails and the device needs to be reset to get it out of a locked up state.
If you update to 1.2.1 with the CLI 1.43.0
particle update command via DFU mode from a Device OS version prior to 1.2.1-rc.3, it will require a reset. After doing that if you had credentials and online connectivity in your device it should boot in safe mode and get the bootloader from the Cloud, restart and be online. If your device was offline, it will appear to be in safe mode but be blinking green because it doesn’t have credentials. You can simply run
particle update again and it should succeed this time, but you’ll still need to give your device credentials to see it connect to the Cloud.
We are currently looking into this issue for users that update with the Particle CLI and it should not affect a device that gets a 1.2.1 user app flashed to it OTA, which causes a safe mode healer update to system and bootloader.