Yep. it is actually largely dubitative.
I don't know, I simply have no idea of the number of device units of the Gray thingy sold, and how much they could have been used, and as well I have no idea on how many crimes involve the access to a (password protected) iPhone, but unless (yet another piece of data I have no idea about) all suspects either use an Android (or however non-iPhone) or give to the police access to the device without need to "crack" the access code, they should be non-trivial amounts.
I mean, I understand that it is a fraction (going to trial) of a fraction (criminal cases involving a phone) of a fraction (suspects not providing access to the device) of a fraction (suspects using specifically an iPhone), still the ubiquity of iPhones and the large number of criminal cases should anyway result in tens or hundreds, while we have AFAICT none or maybe a few actually documented/published about.
The declining of local reporting may be indeed another factor, but there would certainly be lawyers making a fuss (right or wrong) about this or that right violated through the use of this or similar tool.
The contract with ICE:
https://www.forbes.com/sites/thomasbrewster/2019/05/08/immig...
is likely to produce no public court records, just like many national agencies, still the sheer fact that not a single actual court record of its use can be found (at least by me) could mean that the device is not used that much by local police, a few cases should have been brought to the attention of the public.