My wife and I have hundreds, maybe thousands of hours on our on-bike cameras. We use the cameras as a dash cam to capture road incidents, wildlife, etc. When we encounter unsafe situations on the road, we upload the videos to Youtube and share with local officials and the media - both have been effective.
For the best video in 4k, Sony and GoPro work well. I prefer Sony, as it has better audio and the phone app is less wonky compared to GoPro. The phone app is convenient for seeing what your cameras sees and aiming your camera on the mount. Both Sony and GoPro are excellent cameras. For bad weather both Sony and GoPro cases are weather resistant and they also make waterproof cases for extreme sitations - but I never bother with the fully waterproof case.
My wife uses the Cycliq Fly12 CE (front-facing) and Fly6 CE (rear-facing) cameras. The Cycliq cameras shoot in 1080p, which is still excellent high definition, but not as lush of quality as 4k. However, you can still read signs and license plates with 1080p.
The big advantages of Cycliq cameras are they 1) double as a front or rear light, with solid or flashing beams; 2) have very large built-in batteries, so you can shoot continuously on long rides; and 3) are super simple to use. We've experienced that Cycliq offers excellent customer service, when we've had an issue.
I use a small usb battery charger "brick" to power my Sony or GoPro via a usb cable, and can then record for hours instead of the usual battery limit of about 1 hour for Sony or GoPro.
For Sony or GoPro, make sure to use a short, stiff metal mounts to connect to your bike or vibrations can turn your video to jello. K-edge is an excellent brand for mounts. A helmet mounted cam works well too, as your neck is a perfect shock absorber. Plus, when mounted on your helmet, you can aim the camera by swiveling your head. Both GoPro and Sony make mounts using 3M adhesive patches to to attach to your helmet securely.