Works reliably after replacing the power adapter
Conclusions: Like a lot of other people, I initially had problems with mini PC randomly rebooting. I know that the power adapter is a frequent cause of problems with Raspberry Pis, so I replaced its power adapter with a 2A one from Adafruit.
That was 3 weeks ago, and since then I haven't noticed any unexpected reboots. Hopefully that's fixed.
Otherwise, it's a good product for the price. It runs quickly enough for common desktop usage: Chrome, Adobe Acrobat, Microsoft Word all run fast enough to be useful.