Many crimes are not reported to the police, and the crimes most likely to go unreported are the ones that involve neither injury nor property loss, i.e., those that had successful outcomes from the victim's viewpoint.  Likewise, assaults without injury are less likely to be reported than those with injury.  By definition, all incidents involving successful defensive gun uses fall within the no-injury, no-property-loss category, and thus are largely invisible to the police.  Therefore the police never hear about the bulk of successful defensive gun uses, instead hear only about an unrepresentative minority of them containing a disproportionately large number of failures.  Further, even when they do receive a report of a crime that in fact involved a gun-wielding victim, the victim has strong legal reasons for leaving their own gun use out of their account of the crime.

If the number of illegal firearms is to be believed then few victims have the required licence for possession, therefore many gun uses probably involved a crime on the part of the victim.  To conclude that armed resistance is ineffective or dangerous, based on the experiences of this sort of unrepresentative sample of victims and advising victims to not use guns to resist criminal attempts seems imprudent at best, dangerous at worst.