The story has some major twists and surprises, and I won't give any of them away. But I can say that the movie really excels in a variety of intersecting plots that not only include family life on both sides of the border but sharply drawn differences in decency and brutality, intelligence and not so bright, in the the major and minor characters. It's a measure of how well presented a movie is when you can remember minor characters such as an older women who helps a younger woman who's pregnant, and a sheriff's assistant who does the right thing. At the same time, though we think we know the major characters pretty well, our expectations are slapped in ways that keep the story percolating and largely unpredictable - no mean feat in such a well-trodden area.
There was one small part, however, that bothered me, because it presented a conspiracy theory as if it were a matter of confirmed reality. At one point, the people from Mexico being escorted to the United States by a "coyote" spot two in their group in Islamic prayer. A bit later they freak out when they hear them talking Farsi. As far as I know, the idea that political enemies of the U.S. from the Middle East - i.e., would-be terrorists - are entering the U.S. through the Mexican border originates in the over-active imagination of conspiracy theorists and some Republicans in Congress. True or not, this played no role in the movie, and therefore its inclusion in the movie makes no sense - unless the makers of the movie were trying to smuggle in a half-baked political statement.
That said, I would still strongly recommend Frontera, for its great acting and memorable human stories.