It' been a few years since I did (ATC) incidents investigations but FR24 was not used to investigate , we have better tools , real ADS-B and enhanced Mode S ( downlinking of aircraft parameters) . which is available via RADNET on almost the whole of the core area of European continent ,and I suspect the same is true for mainland US ( exculding Alaska I think ) . For out of "standard ground based " ADS coverage , there are other options , Iceland , parts of Greenland and Canada use Space-based ADS-B . There are however still gaps, on the standard terrestrial system notably at low levels and in mountains areas
As you correctly point out , DaveReid UK, FR 24 data is often incomplete and "voids" are calculated. It is also not cerified. It gives a good indications . but not the exact values , Howver , like in Norway or in the Alps, (and probably the US rocky mountains too) some valleys where there is no "official " ADS-B coverage but where a local farmer had puchased a FR24 or OGN antenna on his roof and sends the data, there, yes FR24 can provide good additional info . Especially for SAR .
Note (*) the nterm ADS-B is generically used to mean many things. What FR24 does is not really the ADS-B we used for separation in ATC. Real ADS-B is more than just downloading transponders data. That said , it is fantastic generic tool and , above all , unlike real ADS-B , it is free
