If you want to quickly understand what this film is like - it's like Cloverfield, but in the third world. It's shaky-cam, it's about massive tentacular monsters, mostly glimpsed at a distance, it's a boy and girl going through a danger zone. The premise is that a NASA spacecraft, biologically contaminated, crashed on re-entry about ten years prior. The border area between Mexico and the USA is sealed off, and horrid alien creatures are contained there. A war photographer has to escort the daughter of the owner of his magazine back home.
Very little is explained narratively. The visual 'show' is very good and more or less stands in for 'tell'. You see old graffiti of the monsters, on bombed-out ruins. You see a Mexican TV advert, telling children to keep their gas masks handy. You see F-111's roaring past.
I liked the third world context of the film. I thought it genuinely gave a feeling of what it is like to be in a poor country, as a Western visitor rather than tourist. The protagonists were much less annoying than I expected, less whiny and entitled. In particular the rich girl could have been awful, 9 films out of 10, but I thought she was a likeable realistic woman.
There isn't much plot at all. The concept of the aliens I think is quite well realised. My feeling is we are meant to feel that the containment has been farcically unsuccessful, and that the US are in pretty heavy denial. A lot of reviews I have read suggest the whole thing is an allegory about containment of Mexican 'aliens'. I would say there are quite strong allusions to that idea in several places, but the whole thing isn't a kind of parable about immigration. It's a monster film.