So many of these stories have disputes between what the police claim to have done and what the civilians claim. It seems that it is very difficult to prosecute

So many of these stories have disputes between what the police claim to have done and what the civilians claim. It seems that it is very difficult to prosecute the police on these matters. Would hard evidence, such as video or audio recordings make a difference in the chance of winning a lawsuit or criminal conviction? Or would that evidence get destroyed or be inadmissible in court? In interested to know how the prevalence of always on recording devices might impact law enforcement.