I have the same nuc as you. I managed to update the bios to 025 the first day i had the nuc. As of yesterday i couldn't even get in to the bios. The machine works fine i windows.
The one time i was able to flash a new bios i did so from a usb 3.0 stick.
The only way i now can get in to the bios is by presseing the "on" button for 3 seconds. I tried to flash the 025 bios again but now the nuc just restarts and load windows.
I suspect the nuc has fastboot enabled even though this isn't enabled in the bios.
I couldn't get the NUC to recognise a USB 3.0 stick.
You may want to try with compatible RAM and tested USB drives from the lists linked below. It is also a good idea to use a single RAM stick during the BIOS update.
I managed to get the BIOS to update when I removed one of the sticks of RAM. Also managed to get ESXi to install without issues.
My problem is now only when I have 2 sticks of RAM installed. I have tested both sticks individually and they work fine - just not when installed together. Now to figure out that issue.
Did you check if the memory is compatible or using the appropriate voltage?