The problems start when, not infrequently, they kick down the door of the wrong house and are met by an armed homeowner.
Current law in many states says that killing a police officer in the line of duty is capital murder.
The sense in which this law is bad is that it perpetuates a cycle of violence and possibly enhances it. If cops do have to worry about getting shot when they bungle a no-knock raid, what if they are more inclined to shoot to kill and leave no witnesses?
"Oh, yeah, it wasn't the right address but we found stuff there."
What the law may well not do is fix the underlying problem - the no knock raids.