Factual error: Druig leads several warriors outside Tenochtitlan as it was sacked by the Spanish conquistadores, and they live peacefully in the nearby forest, for 500 years. The forest is of course the virgin Amazon forest, as captions say. Small problem; Tenochtitlan was in Central Mexico.
Factual error: Thena and some Eternals go to the Amazon. The opening scene shows the Amazon River lowlands. In the Amazon they speak Portuguese. But the Eternals speak Spanish to the natives. A part of the Amazon is in Peru where Spanish is spoken, but this is not the area where llamas (shown) typically live. Llamas are highland animals and are in southern Peru, not northern. (01:06:10)
Plot hole: Spoiler; Ajak and 'the true villain' are the only ones who know the true nature of the mission and the fact that the Earth will cease to exist in 7 days. None of her fellow Eternals would know where to find her or suspect that she's dead or that anything is wrong, but the villain makes them find her body on purpose to provide a distraction to keep them busy investigating her death. Provide a 'distraction' to someone who is completely unsuspecting (and actively lead them) is pure nonsense.
Plot hole: Deviants were created to get rid of dangerous local predators allowing intelligent life to thrive on the planets Arishem 'seeded'. He then created the Eternals to get rid of the Deviants once he realised they eat the species they should have protected. Problem is, it is stated that the Eternals go through their extermination routine multiple times. But the 'mistake' can't be happening all over again in a cycle, and Deviants would ruin a planet if left unattended.