The ATF doesn’t call them “ghost guns” like some prejudiced bigots in the California legislature but they did put out a news release on 80% receivers a few days ago. The worst they said was:
6. Can functioning firearms made from receiver blanks be traced?
ATF successfully traces crime guns to the first retail purchaser in most instances. ATF starts with the manufacturer and goes through the entire chain of distribution to find who first bought the firearm from a licensed dealer. Because receiver blanks do not have markings or serial numbers, when firearms made from such receiver blanks are found at a crime scene, it is usually not possible to trace the firearm or determine its history, which hinders crime gun investigations jeopardizing public safety.
7. Have firearms made from unmarked receiver blanks been recovered after being used in a crime?
Yes, firearms that began as receiver blanks have been recovered after shooting incidents, from gang members and from prohibited people after they have been used to commit crimes.
They chose their words very carefully. I think it is very telling they don’t give any numbers on how often traces failed and that actually hindered the arrest and conviction of criminals who committed violent crimes. I would not be at all surprised if the number of people arrested for victimless crimes associated with making guns that didn’t quite meet ATF’s regulations drastically exceeds the number of criminals who evaded arrest because of 80% receivers.