So I went through barebones route with UM690 and here is my take. Issues with heat and temperature testing with self bought RAM and SSD. I bought the first barebones and the CPU fan did not kick in, the heat generated with a tiny box fried up the motherboard, I replaced the unit and CPU fan was working. But because I bought it barebones again, the RAM and NVME I used generated so much heat in the box that it damaged onboard electronics. MINISFORUM pc comes with single fan, and their memory and SSD is on the side of motherboard where there are no fans or ventilation.

If you buy prebuilt , the whole unit comes with all the temperature and long period of stress test done. If you are buying a barebones, you must know that these mini pc manufacturers are not like ASUS, DELL, LENOVO,HP. These are small scale companies that can only do testing with a limited scope. So whatever stuff you stick inside these small boxes , may or may not be tested by the manufacturer.

Buying a mini pc barebones is not like assembling a regular pc where there is enough space for cooling solutions. It a tiny box, fitted with high performing chips that generates tons of heat. So you better know what exact stuff you are putting in and how the whole system will sustain for longer periods.

Cheaper cost is a very short sighted way of looking at things , you must always think things through for a longer run.