There are some major difficulties with cameras in terms of usuability.

1) Battery life
Usually ~2 hrs, which means you're always needing to charge multiple times per week.

2) Image clarity & contrast
Most cameras work decently well in bright over-head daylight. Shadows or bright lights tend to cause significant under/over exposure making the video almost useless.

3) Looped recording
How do you stop the recording after an accident?

Typically cameras seem to capture usable video in about half of incidents. The other times the video is sufficiently blurred or washed out that you can't actually see things like license plates or even make/model of a vehicle. I tried a camera for a while and eventually stopped bothering. My commuting is typically at dusk, and the video was always terrible.