Unrealiability can often be a feature to police, as it is with field test kits and drug dogs. The false positive rate makes them a good tool to provide plausible cover for further police action. By simply running enough unreliable tests or letting the dog sniff long enough, it is practically guaranteed that they can find at least one reason to justify their action.

Same thing with other junk science like bite mark forensics. If you need someone convicted, show it to 10 "experts" and one of them will say they match beyond reasonable doubt.