Good review, keep it up! However, there's always room for an improvement and like pretty much all the other reviewers on Youtube, you didn't really put it trough on any demanding tests for an ACTION camera.
First, to get real picture of the video quality, you need to run it in a fast changing scenery with plenty of details in order to see does the limitations of bitrate create serious bottlenecks in form of compression artifacts.
Second, CMOS sensors have tendency to suffer from rolling shutter effect. What I've found is that the Firefly 8s is quite "old fashioned", to borrow your words, in that regard.
Third, the reason why you couldn't record 4K properly, could be that your memory card is too slow for it (same as I had with one of my cards). Of course, there could be some other issues as well, like faulty unit and over heating, as the camera runs very hot after short usage.