FEMA, having, perhaps, bumped up against hard reality more often than any Federal agency except the National Weather Service, warned going in, it's a test. They were looking for unsuspected weaknesses.
Found 'em. Here in de Hoosier Hardtland, we got the tricky part just fine: the blind-sent data bursts chortled along and the text it's supposed to make pop up on local station TV screens did just that. But the easy part, the 1920s-technology where a fellow in a far distant city speaks and it goes to broadcast stations and we all hear it? Failed. Thirty seconds of between-stations static is what we got -- and early word in the biz is, that's exactly what Uncle Sam sent to the big AM station that serves our area.
In other regions, the voice message was garbled, "echoey," or had a lot of noise. Early word is the entire state of Oregon received.... nothing at all! In some cases, stations that thought they were ready learned otherwise; but that was far less common than high-level failures at the originationg points serving entire regions,
You will, however be relieved to learn that in New York City, where "remain home, remain calm" is about the only possible emergency information that won't result in panic and gridlock, the system appears to have worked fine.*
Before we go off in a chorus of "Ha-HA!" like the kid in The Simpsons, I have to point out that nearly all the state-level systems (which are tested monthly) worked A-OK. If FEMA sent out noise or garble, they faithfully relayed it to a puzzled public, in exactly the same way they relay the clear and understandable monthly test message and weather alerts. Sure, you're online with Twitter, Facebook, blogs or news or listening to an iThing, but if you do have a TV or radio on and there's a typical state or local emergency -- tornadoes or locusts, tidal waves or wildfires -- the message will get to you.
However, if the Postal Service goes belly-up or Washington, D.C. goes down like Atlantis, I would not expect the EAS to carry the warning. At least not until they figure out what went wrong and go about the painful process of fixing it.
You might take some comfort that on 11 September 2001, when a national-level emergency did take place, the news was disseminated rapidly. Not by the government but by the various companies that run for-profit networks all day, every day: ABC/CBS/NBC/Fox/CNN etc. all jumped on the story as it happened -- and even pushed their scheduled commercials aside to do it. There wasn't any question of the message making it to you; it had been reaching you 24/7/365 to hawk soap flakes and cornflakes already and when the stakes suddenly got much higher, it was already in place, working.
Funny, that. (Oh, Adam Smiiiiiiiiiith....?)
* I'd like to mention that in the two most recent actual emergencies to afflict New York City, the World Trade Center attack and the 2003 Northeast blackout, NYC residents did extremely well, stepping up to the situation and doing what needed done, a far cry from the kind of chaos predicted in fiction.