I am ashamed to admit, but I was stupid enough to fall for this. I saw a "4K action cam" for ~$30 and thought how bad could it be? If it works at all it's good deal, right? I was not naïve enough to hope for real 4K, I expected the 4K to just be some interpolated crap, and would have been happy with 1080p, or even a half decent 720p.
Well, the camera arrived, I quickly went through the settings, cranked it up to 4K of course, and shot some test video just to realize that the image quality is utter garbage. Interpolated for sure, but not from 1080, or even 720, but rather from 360 (or maybe 480).
Anyway, I played around with the settings a bit more, tested some other modes and then, about half an hour into me playing with it, the 128 GB Class 10 microSD was bricked. This was more than three years ago, so the price of the card was significantly higher than the camera.
Lesson learned.