I have a SPOT and it transmits just fine. With a weak transmitter I'm sure it has to retry at times but my wife can follow my track on trips and get messages I send for change of plans ("winds up, delaying return trip"). Generally the unit isn't under water much for that to be a transmission issue.
Sorry, no info on other types of units. Some like traditional EPIRBs are more sure fire for rescue anywhere in the world. But for those in less remote areas the ability of other PLBs to let friends follow you on a map or get less urgent messages can be a bonus.
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