I have had exactly the same issue.
If I try and boot the NUC with a Sandisk Extreme 64Gb USB 3.0 stick installed, the machine displays the "Intel NUC" logo for ages, and eventually when you get control of the NUC, it won't allow booting off that stick.
If I start the NUC with no USB 3.0 stick installed, it brings up the boot prompt almost instantly when powering on.
As soon as you remove the USB 3.0 stick that you've inserted at boot time, the NUC comes to life.
So it seems to be hanging on USB 3.0 sticks.
As the original poster pointed out, no such problem occurs if you insert a USB 2.0 memory stick.
The NUC can recognise USB 3.0 drives/sticks only after start-up. So if you have one installed at boot time, which you'd do when booting off the USB stick, then it will fail.
I have tried all the BIOS versions from September to November, and all have exactly the same problem.