The aliens, we are told, are brought back from Jupiter's moon Europa to earth by NASA. From what we know of Europa, it has a surface made of ice, which possibly covers a liquid ocean. This may or may not be water and may or may not be capable of sustaining life. The average temperature on Europa is some 170 degrees below zero.
So we are assuming that life can be sustained on Europa. That is fine. Considering the conditions under which this life must exist, though, are we to also believe that it could not only survive, but seemingly flourish on earth? On dry land? With the temperature so vastly higher?
There are many other reasons why the creatures may not survive, such as the entirely different environment and elements for one thing. The difference in gravity for another. Most of these things I can accept that it just happens to be Ok, but I cannot imagine that creatures designed to live in an ice-capped ocean at temperatures closer to absolute zero than our own, could possibly be versatile enough to appear completely at home in a Mexican jungle.
Otherwise, it's a good movie.