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. I would have a belt and braces 406 beacon as well.
406 beacons also need a clear view of the sky. Actually, they need a clearer view than that required by Iridium and Globalstar based devices. Re-read my post on this above.
There is absolutely nothing substandard about the Iridium and Globalstar based devices except for the lack of a G-switch. Other than that in most ways they are superior to 406MHz devices. Again the best solution is to have both.