The point of the tactic (part of the Reid technique,) isn’t to get evidence. It’s to get a confession.

The problem is it works by scaring the shit out of the subject in the hopes that they confess to the first person to show some sympathy.

Which, it does work. People subjected to it will eventually confess. The problem is… they may not have actually done it and they’re afraid of all the things bad cop is threatening them with.

It’s also very easy to manipulate someone’s understanding of what happened when they’re in the high stress of an aggressive interrogation.

And just for the record- the cops are allowed to lie to you (at least in the us.), so they can tell you they have evidence they don’t have etc. which is why you need to avail yourself of a lawyer… and say nothing to them.