Here is a great example of the difference in ADS-B coverage among the different flight tracking sites. ADS-B exchange has this USAF RC-135V Rivet Joint last seen over three hours ago about 150 miles south of San Diego. However, looking at Flight Radar 24 we can see that it currently in international airspace over the Gulf of California. In good position to gather intel on Mexican Drug Cartels.
https://globe.adsbexchange.com/?icao=ae01c7
https://www.flightradar24.com/38fdf9ab