I liked the movie. I think the greatest tension in the film is seeing a pair of families at turns unraveling in the face of breakdown and then coming together in their moments of need. You don't need to be shown massive waves of death to see doom. Instead, you're given glimpses from the normal life to the surreal effect of society's breakdown on a personalized scale. Fires in the distance, nature out of balance, roads choked with seemingly abandoned vehicles. loss of media and online capability.
The lack of explanation actually heightens the tension. We're given a couple of hints, but even the stark pronouncement at the end is only an opinion. This would be the reality if our society suffered a longterm loss of communication. I think the downside is that the movie could have been longer. It feels almost too pat with the ending, so lesser in the conflicts in the film won't be explored to their conclusion.
OK. One other point I'd like to make for anyone looking to online reviews to make up their mind whether to watch... you'll note the very low overall rating for the film, and you'll wonder why. Well, all you really need to know is that you will find only 3 or 4 originally written bad reviews, and the rest are copy-paste variants. In other words, right wingers saw that a critically acclaimed film had backing by the Obamas and lost their cool, so decided to review bomb the thing. Thus once again, right wingers exploit a system to punish what they don't like. It's doubtful the majority of single-star reviewers even saw the film.