ADS-B data is sent along with various quality indicators. High-quality ADS-B OUT systems send higher-quality indicators than the systems installed in most GA small aircraft.
Examples of these values are SIL, SDA, NACp. These are set to "0" in many transponders. This has the advantage that cheap GPS data sources can be used.
In Europe, well over 50% of ADS-B transmitters in lower airspace (approx. below FL100) send ADS-B data without quality indication (as of spring 2021). This does not mean that these data are of poor quality, but that it is unknown which quality they are. This is actively supported by the authorities (e.g. EASA via CS-STAN) because the effort to send better data is high and the safety benefit in this area of aviation hardly exists. To put it bluntly, we in Europe prefer that traffic participants send data than that they do not send any.
Systems that only receive ADS-B signals with high quality only process a fraction of the traffic data of the AT-1, in Europe also not even half of the ADS-B messages. This is actually not useful for ADS-B IN here. Such systems see about 5% of the total air traffic, an AT-1 sees 80+%. The AT-1 processes all ADS-B data (plus FLARM and transponder data).