Just so happens I did some more research this evening and this is not a NUC defect at all.
The OS I am running is vSphere 5.5 and I found a couple of people that make reference to the partition type ESXi 5.x creates by default. Apparently, it defaults to GUID Partition Table (GPT) partitions which doesn't seem to be very friendly to USB thumb drives and "legacy" mode in the BIOS when attempting to boot. I learned we can force vSphere to use the traditional master boot record (MBR) at the time of installation. If you enter the boot option prompt, enter a parm called "formatwithmbr" which tells the ESXi installer to use MBR instead of GPT.
Tested several times and works great - problem solved!
I won't go into a lot of detail as you can search for existing articles that spell it out step by step. Use a query similar to "vsphere usb formatwithmbr". That should get you down the right path.
The NUC is well on its way as a low power server for a small test env. As originally stated, I am very pleased thus far.
Hope this helps someone else.
You're awesome, thx for posting this. Been searching in the wrong direction, until I found you're post.