Well it sounds like you have been pretty diligent on testing this hardware. We ( i ) have the NUC5i on my campus and they do work (with the exception I don’t boot though the FOG server on an operational basis. We require the techs to press F12 to get the boot menu for imaging to avoid an accidental imaging event).
Just as a point to note, if you did something with syslinux then you have a problem. FOG does not use syslinux since the early days. iPXE is now used for the boot loader. What are you setting the dhcp option 67 to?
This sure does sound like an exit mode issue. Are you changing the bios exit mode in the host definition or the global setting?
Also does that NUC have a personality selector? Some allow you to define Win7,Win8,W10, Linux and we found that it IS important for that personality to be set correctly. This is set in the firmware and not the OS or FOG.