Could you elaborate on how many USB devices and hubs, if any, you have connected to the NUC, and on what ports they are connected? Also, what are the make/model/part# of USB sticks have you tried?
I had some problems hanging at POST until I grouped USB devices not needed for BIOS/boot on to a single port and set that port to "No Detect" in the BIOS. Boots like a champ now.