- How accurate are these flightradar's data?
Flightradar24 relies often on ADS-B transmissions from the aircraft. Depending on the aircraft the accuracy of the transmitted ADS-B position varies from very accurate to not accurate at all. This particular A319 belongs in the latter category; the source of the data was the Inertial Navigation System.
- How common is for pilot to miss runway in heavy fog?
Quite common. Depending on the aircraft, the flight crew and airport navigation facilities the aircraft can land in zero visibility. Nikola Tesla airport has Instrument Landing System (ILS) CAT IIIb on runway 12 (where you landed) which means that the navigation equipment can support autolanding. In this case the decision height can be at the runway, the minimum visibility at the runway is 50 meters. At the moment there is no Notice to Airmen (NOTAM) published regarding maintenance of the system so I assume it is operable.
I don't know whether this particular aircraft and its crew are approved for ILS CAT IIIb operations.
If either the airport, the aircraft or the flight crew cannot support autoland, then the runway has to be visible at a predefined height. If the runway is not in sight at this height the flight crew will follow the missed approach procedure.
- Is there a reason to be worried while flying 50 feet above meadows in heavy fog? Possibly with malfunctioning navigation
No there is no reason to worry. This is normal operation, it happens everyday somewhere in the world. The aircraft is guided by two independent guidance systems that cross check each other. If there is a discrepancy between the two, an alarm sounds and the approach is aborted.
The navigation system on the ground is carefully monitored, if anything malfunctions the system will raise an alarm. The area around the runway and especially around the antennae of the navigation system is kept clear of obstructions when the airport is under Low Visibility Procedures (LVP)