So now it has been running for 4 days and 9 hours and never did I get a freeze during this time, so that's very positive. Now I'm shutting down, but I just wanted to post my positive experience with this long uptime and also get the chance to explain more about my problems with freezes.
I've received the help from here that this OS is not supported on this NUC(DN2820FYKH) so I'm looking for community input, if others have had similar experiences like I have.
First the positive news. Typing the "uptime" command I've now enjoyed 4 days and 9 hours both watching Dota 2 tournaments and playing some Dota 2 games myself on the unit among other games, watching videos + more. Since I know Dota 2 does not meet the hardware requirements I've lowered the settings ingame to the lowest possible, and it works pretty smoothly, so I can enjoy playing the game. During this uptime I got no freeze at all, because a freeze requires me to reboot the computer, by having to hold the power button until it shuts off. A "soft freeze" may dump me into the login screen, but then I still keep my computers uptime, that's not what I name a freeze, but rather just a "hiccup".
Some info on my installed system. It's Ubuntu 15.04 with default "BIOS Version 0048" settings, excepting the only change being me disabling UEFI boot.
The HDMI is connected to my VGA screen with a HDMI -> VGA converter.
When it freezes it never lets me get such great uptime as I have now 4 days and 9 hours. It would never let me go beyond 1-2 hours uptime before the freeze.
I have two kinds of freezes.
* One is "matrix-style" where the screen is filled with green stuff that looks like the intro to the movie matrix,
* 2nd is a normal freeze where everything stops moving, including the system clock HH:MM:SS.