Coverage is somewhat limited in certain parts of the world.
Sure, middle of the Pacific isn't great. I think someone venturing that far would do well to get a real EPIRB. Globalstar network has two bands - the duplex band used for phone service is not doing great (who knows, they are working on it - they are the only financially viable option anyway, Iridium is just out of this world in terms of pricing). The simplex band which this messenger is using is actually in good shape - they've been using it for remote telemetry and such, and just figured out the messenger idea (amazing, took them what - 10 years?
Coverage is certainly available anywhere vast majority of boaters would ever be - even across Atlantic in most places and certainly hundreds of miles offshore. These guys on Chesapeake would have benefitted from the device for sure.
It is a bit picky about sky view (though it turned out to be a non-issue on the boat, as I hoped it would). The good thing is, you get to test the connection all the time, using tracking messages so there is an easy way to know the system is working end to end. And if you send a distress signal, it also goes to a list of emails or phones so presumably if the guy at emergency response center is asleep - someone else would intervene.
Lack of these options is what always worried me about EPIRB. There is no way to test them now, as there is no "test window" or anything, so when the time comes, you push a button for the first time really and hope that it works. As a "computer guy" I don't trust systems whose first use is when it can't fail.