In my country the one and only time they've ever used the system at its highest priority level, it didn't work for half the phones. And a good 20% of the people got the alert over an hour late.

Last night they sent out an alert at low priority because there was a "risk to life" weather event. But I guess they didn't want to worry me too much, you know because it would be terrible if I took action of some kind. In order to get these alerts you have to have the weather widget on your home screen, presumably iOS users just don't get told.