From what I've seen within the United States, virtually every case where some "sexual depiction of imaginary children" (excuse me for the jargon, I don't like jargon myself) has turned up at court involved people who the police were already charging for committing some other crime.

This isn't because the police aren't investigating either. The police have lost interest many times when it came to light that it is just that. It is also well-known that trying that would violate the First Amendment.

The First Amendment has even been said to have been brought up. The federal government even publicly admitted last year that it is generally protected by the First Amendment.

Legal scholars from Stanford also noted this year that if it were brought up in court, it would likely fail to pass muster.

Other than that, it's just not effective at combatting crime and wastes time which could be spent on real criminals.