Motherboard built a database of over 500 iPhones that law enforcement have tried to unlock. Many of them weren’t able to be unlocked at all.
Out of 516 analyzed cases, 295 were marked as executed. Officials from the FBI, DEA, DHS, Homeland Security and Investigations, the Bureau of Alcohol, Tobacco, Firearms and Explosives were able to extract data from iPhones in investigations ranging from arson, to child exploitation, to drug trafficking. And investigators executed warrants against modern iPhones, not just older models.
As mentioned, this provides useful data instead of the usual anecdotes. You can find the database here.
Check It Out: A Database of 500 iPhones Cops Tried to Unlock