There is much confusion about the question: do the i5 and i7 really support 1866Mhz memories, or do they run those at 1600Mhz? Is there really a difference with the i3 on this point?
It is specified by Intel that the NUCs run memory at 1600MHz at the most:
"If this is the model that you're talking about NUC5i7RYH the memory seems to be fully compatible, however since the NUC only supports DDR3L-1333/1600, if you run it at 1866 MHz it might not work properly or it might not work at all.
You can check the compatibility of the NUC5i7RYH here ARK | Intel® NUC Kit NUC5i7RYH"
But according tot Legit Reviews, the (CL10) Kingston 1866MHz performed slightly better than the 1600MHz model and in the NUC BIOS the 1866MHz was being displayed:
The latter link is showing a screenshot of the BIOS with "1867Mhz". I'm not technical enough to say this means that the 1866-ram is only recognized, or that the NUC is actually running it at 1866MHz.
And this reply from Intel Customer Support (June this year):
"By reviewing both specs I concur with you in the sense that both are pretty much identical.
The main difference between models is the programmable latency; which does not represent a perceptible difference for a NUC unit.
The list of memory modules tested during development serves as a guidance for compatibility; but some newer memory modules that have the same specs might work as well and will not be listed in the provided website.
With this in mind, we can only recommend the modules we have tested; however, you can try on different modules with the same specs."
Can someone around here provide the answer?