I have had major problems with Ethernet too. Newly bought i7 NUC was otherwise working fine (and very fast) but I could not install drivers for Ethernet and a highly technical friend worked for hours on the machine and advised me not to rely on Wireless because I will then have no options to sort matters out from scratch if there are problems with the machine in the future. Friend finally concluded that there must be a (rare) Ethernet hardware failure. So I have returned the machine and got a replacement and I am again going to install W7 and upgrade to W10. I will advise whether Ethernet works with the new NUC.
Thanks for letting me know David
Intel update Utility killed my Bluetooth after I updated it and I only had a Bluetooth mouse at the time, had to reinstall windows to fix that