The criticism is usually that the, "cop fucks something up," elements usually are constructed so the audience feels like they're on the cop's side. Cop does something illegal that jeopardizes the case, and the audience feels like, "bull shit, this guy deserves to go to jail, it's not the cop's fault the system is broken."

Basically, it's real easy to walk away with the impression that incredibly important legal protections that ensure justice is fairly doled out are actually preventing justice from being served.