FYI, posting for future reference.
When putting device into uBoot mode, it is very important to hold the reset button down at least 1 second AFTER to 2.4 light comes on. During this process, once you turn on the device, holding down the reset button, the power led will flash 5 times, then the 2.4 led comes on, release the reset button at least a second after this happens.
Doing the above, I was able to successfully update the device, every time. There is no "Update complete" or even a progress indicator, I left it for 4-5 minutes after the upgrade process began before power cycling.
Remember to switch local machine IP back to DHCP from manual after upgrade completes.
Does anyone know how to export settings for easy recovery if this happens again?