We gotta understand that while the travel of Álvar Núñez Cabeza de Vaca is based on a true event. On something that indeed happened. Back in the XVI century the concept of what was the "truth" and what should be understood as the "truth" was very flexible.
e.g. Bernal Díaz del Castillo who wrote (actually he only dictated it) the most famous chronicle about the conquest of México, did it decades after the events he narrates took place, when he was already an old man, and only because he wanted to retain his lands in Guatemala. Therefore he magnified his feats and minimized his mistakes.
Same goes for Álvar's chronicle or for any other account from the time for that matter. Now, I am not saying all they wrote were mere "lies". Not at all. But you should take with a huge grain of salt everything the conquistadors and adventurers wrote.
Besides, most of the tribes that Álvar met during his travel, no longer exist. They were decimated by the spaniards, then by the frenchs, british and finally by the yanks. Since they didn't have any sort of "high culture" (temples, palaces, sculptures, paintings) unlike the nations from Central and Southern México (Maya, Aztecs, Teotihuacans, Toltecs, Olmecs, Zapotecs, Purhépechas, and so on), therefore much had to be "imagined" by Nicolás Echevarría.
Now since Echevarría was both a filmmaker AND ethnographer, he knew quite well how the former tribes of Florida, Louisiana, Texas and Northern México should have looked and behaved.
Anyway that's not the point of the movie. The point was to show the clash of civilizations that occured between XVI to the XIX centuries across the Americas. A clash of worlds, a clash of customs, beliefs, traditions, etc. In one word a clash of different "cultures". Depicted in the movie by the impossibility to understand the "others".
Since the europeans couldn't understand these new lands and its inhabitants, they feared them and treated them like wild beasts that should be tamed by the force using as a weapon the christian religion (spaniards), or simply wanted to exterminate them (british and yanks). Portuguese were somehere in the middle.
Nowadays everybody knows (or at least should know) that such kind of policy: Indoctrination OR extermination (shared by all the european settlers) plus the diseases, caused the biggest holocaust the humankind has ever known (between 90 to 99% of the native americans were exterminated between XVI and XIX century).
"Cabeza de Vaca" is a remarkable film, visually impressive. And as some other poster correctly said: It's the right antidote to that abomination of a movie called "Apocalypto". The latest stabbing to the native americans and their culture.