Are you (or this customer) regularly cleaning this NUC? That is, regularly using compressed air to blow out any dust buildup? You should be doing this every month or two.
I carefully examined the Nuc and could not find any evidence of a fur or dust buildup of any kind. Cleaning the Nuc with a can of compressed air did not yield any noticeable dust.
Ok. What kind of temperatures are you seeing? Can you run the Intel Processor Diagnostic Tool and upload the report generated? You can download this tool here: https://downloadcenter.intel.com/download/19792
I don't use compressed air to clean a computer. Many times you get moisture. Further, the best way to clean a NUC is to open it up and vacuum out the dust that collects on the opening(s) of its fan. You need a micro vacuum attachment kit that fits on the end of a regular vacuum. I have the 5i7RYH, and it started acting up because of dust build up. It took years to get this way. This only needs to be done every 6-12 months in a relatively clean environment. I do not recommend putting a computer on the floor on top of a rug or carpet, as there is too much potential for dust. When I first got my NUC, the sound output plug was bad and intel replaced it. Now that I am at 39 months, the video is failing. Intel suggested that it was the adapters that I was using, but this does not explain the following. If I touch either the mini- HDMI or mini Display port plug, both monitors flicker or go off. It isn't the adapters since the two plugs are on opposite sides of the rear panel.
If the NUC meets your performance demands, it is a good choice if you want to stay up to date on your computer. With a three year warranty, it forces you to replace it after that time period when something you need fails. Chances are that you can take your SSD drive out and put it in the new model.
I most certainly do - and I purchase compressed air that is specifically designed for use in cleaning computers and do not introduce any additional moisture. Further, I have found, over the years, that the micro vacuums are completely useless. I have at least a half dozen that I have tried - and discarded.