I'm running Linux on a UGREEN NAS without a problem from the OS perspective. On AOOSTAR even Windows 11 ran fine after digging for appropriate drivers.

Where most of these machines suffer is the default UEFI config and the cooling. The UGREEN luckily gives the option to dig into the advanced UEFI settings (they just left the entire setup open in "engineering" mode so every option the firmware gives is there to tweak) and optimize what the manufacturer defaults, change the power saving options, turbo and power limits, etc. This makes a huge difference.

But the CPU cooling is still sub-par and there's no off the shelf replacement. Most of these smaller Chinese NAS brands I tried have this issue, they take very decent hardware and slap on them the crappiest accessories like cooling. You get a hobbled CPU, always pegged at low clocks because the temperature spiked to 99C. Sometimes the disk cooling isn't too well conceived either and the disks bake at 45C idle but these can usually be compensated if you're willing to take the noise.