I bought a NUC6i3SYH at the end of last year to use as a home server. It works perfectly except that once every few weeks or so, it freezes up and requires a hard reset. The freezes have no apparent correlation to heavy load, cron jobs, network traffic, heat, or anything like that. Once the machine is frozen, I cannot log in either by network or by plugging in a keyboard, mouse and display (the machine is normally headless). This has happened maybe half a dozen times by now.
What's especially strange is that when the machine crashes, it takes down my internet connection as well. The NUC is plugged into a router (WRT610N). The router usually stays up, but it cannot send packets over the WAN port and eventually loses its IP address. Unplugging the NUC's cable alleviates the problem -- the internet comes back up within seconds -- but I don't think it fixes the NUC. Once my roommate reported rebooting only the router during an outage and the NUC was accessible afterwards, but I haven't been able to reproduce that so it may have had some other cause.
This happened with BIOS 028 and 033. I'll put 036 on it soon unless you all have a better idea. 16GiB (2x8) Kingston HyperX 2133 RAM, SATA SSD, Ubuntu 15.10, nothing with graphics on it, usually lightly loaded. I ran memtest86+ on it for like a day, with no errors. BIOS settings are close to default -- I think I set it to come back on after a power outage and that's about it.
You could try installing the latest BIOS version, but bear in mind that Intel does not test and validate Intel® NUC on Linux; however we know that a lot of NUC owners are using it successfully on many different Linux distros. Intel suggests you check your Linux distro's website and forums at Official Ubuntu Documentation for peer assistance with this issue.
For further reference visit; Supported Operating Systems for Intel® NUC Products.