This is changing with dash-cams. If the other person has a dash-cam you are not free to do unreasonably things and then weasel out of being on the hook on the basis of the front of their car hitting the rear of yours. Of course, if there were witnesses this has always been the case.

If the dashcam shown the driver following too closely and failing to brake in time, I'm not sure a dashcam will help. But sure you can't reverse into someone.

Really? In most states and EU countries its not. It's assumed to be the person behind's fault at first, but that's a heuristic, not a necessary assignment.