I do say and think a single front camera are good for just about anything that happen, at least even if it is off camera it can prove that it was not your fault as your video have you driving along in accordance with the law.
BUT ! i do still recommend a 2 channel ( Front & Rear ) if financially possible.
Back in the day before 2 channel systems became a option, there was talks here and there about redundancy in having 2 single cameras, and 2 cameras in 1 unit is just more prone for problems.
And it is also true that with the 2 channel systems we have gotten more problems to deal with but rarely issues pertaining to the system itself ( what is on the PCB )
The additional / new issues that have popped up, at least as far as my memory go seem to be:
1: erratic camera performance in regard to failing cable or loosened connectors on rear camera wire.
2: RF noise issues hitting some people with SAT or DAB radio.
This is why nowadasy when people come with dual system problems, in addition to the mandatory ( test memory card / try other memory card ) there is also now ( try and unplug the rear camera to see if it work better then )
If someone made a SWEET 4K camera
i would snag that up over most / any dual system.