Whilst the coverage from the Iridium satellites is impressive, your tracker still needs to have a clear view of the sky to see them so not infallible.
The difference is that the tracker shows the track to the accident site even if - for one or the other reason - it stopped working after the crash. So it still served its purpose even if it died in the crash.
Whereby the ELT is silent until the crash happens, and then only works - for one or the other reason - in 1/3 of the cases.