Are you using the same brand and model of RAM for all the units? If no, a compatibility issue should be out of the question.
Please check and proceed accordingly with the following:
1. Test the RAM from the two working units on the non-working units, test both DIMM Slots
2. Test the RAM from non-working units on the units that are working fine (to find out if the RAM is faulty)
3. Use the power brick from working units on the units that are not working (to check if the power supply is dead)
4. Please do as in steps 1 & 2 but with the SSDs this time
5. Test the known-to-be-working HDMI and DP cables, with the units that are not working
Let me know how it goes in order to further assist you if required
I cross-checked again and identified the RAM as the problem. Interestingly, all my units do work with 2133MHz RAM, but only some of them work with the 2400MHz RAM. It's even more strange that for one unit the 2400MHz RAM did not work before, then I switched back to the 2133MHz RAM, upgraded to the latest BIOS firmware, installed Linux etc. and then after switching back to the same 2400MHz RAM module, it worked! But just this unit. Two more units worked with the 2400MHz RAM out of the box.
We now purchased a couple of 2133MHz RAMs which worked before to solve this issue. However, this unit-individual behavior is a bit strange.
This is not strange at all. In fact, it should be expected. As has been stated, Intel does not officially support these faster RAM speeds. As a result, they do not formally design for (though they do try to build in margins) nor formally test with them. When you overclock memory (and using anything above 2133 should be considered overclocking), you are essentially increasing the clock frequency used on the memory channels. As you increase the frequency, you are also increasing the noise on the bus. If this noise - which can (and will!) vary from one individual NUC to another - reaches levels where it prevents the recognition of the data being clocked across the bus, bit errors - and hangs if they aren't recoverable - can occur.
Buy 2133MHz DIMMs. Faster DIMMs are not going to make enough of a difference in performance to make the (painful) matching process worthwhile.