USA study looking at accident rates for GA planes & air taxis 2013-2017 found that the aircraft equipped with ADS-B IN were 48-53 % less likely to have an accident and 88-89 % less likely to have a fatal accident.
That study shows the effect of a third party supplementary system that uses the ADSB system to deliver additional information to apps installed and displayed on ADSB-In receivers. ADSB-IN in and of itself is not going to produce those results.