UAT 978Mhz and ADS-B 1090Mhz FAA ground coverage should be the same. I believe that each ground station receives and transmits on both frequencies.
Flightaware receiver coverage is going to be much lower. We only got decent UAT 978Mhz receiver software recently. Not many people will have gone to the trouble of installing it, especially if they are interested in commercial traffic.
Air coverage within the same frequency should be more than enough for see and avoid(as I stated above). ADS-R/TIS-B, if in range of a ground station, will provide everyone UAT, ADS-B and some radar traffic. There is definitely a possibility that you won’t see all aircraft if not in the range of a ground station. This is also the case for aircraft that don’t have their transponder turned on (It is not required everywhere in the US or in many other countries).
978-Out, in areas without ground coverage, could mean that 1090-in/out aircraft won’t “see” you. It is an annoying part of the dual frequency system.
