At least, in areas where such sites have good coverage they may actually get closer to the ground data than the satellites are capable of picking up.
I don't mean to dump on doing a quality analysis, just that the quality of data is not as drastically different as you're suggesting.
Generally data is received every 5 seconds. In cases such as Lion Air, they downloaded all the granular data from nearby receivers and provided some analysis in their blog.
They also made available the raw data for the public to download:
https://www.flightradar24.com/blog/flightradar24-data-regard...
Every flight broadcasts this data kinda like GPS and all you need is an antenna to receive it. In fact the tracking is basically done via ground-based GPS(ADS-B) by measuring the time delays between different receivers in the vicinity. The problem is that there aren't many hobbyists in those parts of Africa so coverage is sparse. In America, its pretty spot on in most locations. A USB adapter can surprisingly pick up planes as far away as 400 miles.
https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/
E.g. you can get an SDR with built-in ADS-B specific low-noise amplifier and SAW filter also for $25. (This can't do general purpose stuff though, unlike the one linked by parent.) Or you can get their general-purpose SDR and their broadband LNA for $40 total.
No, position (likely received from plane GPS) is just openly transmitted in ADS-B frame.