While I can certainly see its benefits in cases like this. It also worries me that falsely accusing someone of murder becomes ever so easier when courts start to trust these devices more and more in the future.

For example, a murdurer wanting to stick the charges on some other person could steal that persons devices (phone, fitbit, etc). Wear them while commiting the murder. Then return them, without notice if done at night or something.

The other person would then have a very hard time proving that he/she did not in fact commit to the murder, simply because courts have started to rely on these devices too much.