Don't go into the movie expecting an action packed blockbuster, that's not what this film is aiming for in the first place. The film can almost be seen as an allegory for immigration, societal imbalance, war, foreign conflict, ect. Those with power and resources make it increasingly harder for those in need to get said resources. For example, you see that the only safe way across the border to America is a $5000 ticket for a ferry across the gulf of Mexico, but for the everyday family seeking refuge they can't afford it so they're forced to risk the lives of their family by crossing the border through the infection zone in order to try to reach a safe place to call home away from the bombings and creatures. The film isn't supposed to be scary, but instead it's supposed to be a more realistic, melancholic, and down to earth depiction of how tragedies are viewed, those not in direct danger are acting like it's just a far away problem they don't need to deal with so they ignore it. It also talks a little about how sometimes the only way people can make a living is to profit or benefit from tragedies.
Spoiler:
A bit off topic, I do find the fact that the opening scene and last scene of the movie are directly linked since the opening scene shows us what happens to our protagonists after the military rescue them. I also like that they depict the "monsters" as animals rather than evil creatures, they're just trying to live. And the governments don't care about the people so they drop chemical weapons that endanger citizens to get rid of their "enemies."