As expected, trying to flash the iMac's firmware onto the NUC was rejected by Intel's updater because of the incorrect public key (Apple uses a different key). In safe mode, you can bypass such checks and force the firmware to be flashed. This is a bad idea. Even though a bad Ridge firmware will end up in safe mode where you can attempt to flash another firmware, the .bin also contains the TI power controller firmware. The power controller manages the reset signal to the Ridge and once that is bricked, the Ridge will not even turn on, much less enter safe mode. Thankfully, we can use our escape route to recover the bad flash. If we look near the end of the update .bin, we see that the Mac's firmware has the string TPS65983 HW FW0005.56.00 ZACE2D3 while the NUC's firmware has the string TPS65982 HW FW0001.12.06 ZTBT1. They are different hardware!