The ADS-B system is only as accurate as the location data source it uses. In the US the 2020 ADS-B mandate requires that horizontal location data be from a GPS source with minimum performance standards set out in 14 CFR § 91.227.1
The aircraft in the Lamia crash in Colombia had ADS-B out that was based on an inertial system resulting in their ADS-B track being off from their actual position. It was off by 2 km when the flight began and drifted further during the flight, possibly being off by as much as 4 km by the time of the accident.2
Although ATC radar will still get an accurate location from the transponder, other aircraft might be using the data for traffic advisories through a system like ATAS. Using a system that could be sending inaccurate data has the potential to be dangerous.3 No info would be preferable to incorrect info. Is there any reason to install an ADS-B out system that does not use a GPS data source?
3At this point there is no reason to believe inaccuracy of location data had any bearing on the LMI2933 accident.