>They use a GPS receiver to identify your location and transmit that to the satellite.<
it is not compulsory to have GPS. The primary location is obtained by doppler method when received by low orbit sats. (This does not apply for geostationary or navigational sats) This is needed even when gps receiver available, as there will be considerable delay time until this can deliver any position data.
Cannot speak to PLBs. For iR devices, the GPS information is the only location information available. You can force iR devices to transmit without a fix, but you must affirm your desire to do so.
In all of my experience I have never had trouble sending an Inreach message, sometimes it might just take 20-40 minutes for it to send, usually it's 1-3 minutes. I've sent hundreds of messages and used it in very remote areas of BC and Alberta, with a large portion on my GDT thru hike.
Iridium provides pole to pole coverage. If there are problems, it's because of a poor sky view.