On the DVD, George A. Romero explains that his sixth "Dead" film can be seen as the second film of a trilogy that is already complete. This second trilogy begins chronologically with the second film released of the three, Diary of the Dead, and ends with the first film released, Land of the Dead. They have a character, or at least a performer, in common: Alan Van Sprang, who plays "Brubaker" in Land, an unnamed Colonel who hassles the video crew in Diary, and the same character promoted to the lead in Survival. The new film includes a flashback establishing "Nicotine" Crockett as the same character we saw in Diary, though he's somehow been demoted to Sergeant since then. Sometime in the future, presumably, he'll change his name to become the character glimpsed in Land. This is all very interesting to know, but it isn't essential to appreciating or critiquing Survival, since Crockett is more or less a straight man for his fellow soldiers and the people he encounters on Plum Island, Delaware.
In a way, Survival is a reversal of Day of the Dead, with military folk as the relatively sane ones entering a domain of madness. The similarity extends to an island clan's efforts to tame and train their zombified relatives. That clan, the Muldoons, have driven their old rivals the O'Flynns off the island because patriarch Patrick O'Flynn took a zero-tolerance approach to zombies and the infected. O'Flynn goes online to lure survivors from across the country to Delaware, where he expects either to recruit them for the retaking of Plum or simply to roll them for wealth and weapons. After attempting to ambush Crockett's little band, O'Flynn ends up their ally as the soldiers discover the atrocities committed by the Muldoons. It becomes obvious soon enough that there are no good guys on Plum, except perhaps for O'Flynn's estranged daughter, whose survival after her father's exile is thrown into question.
The Big Country, especially in its conclusion. By now, Romero is clearly having a hard time taking the zombies seriously. They're as gruesome as ever when they make their customary final rush (and when Romero reverts to practical gore effects after economizing on CGI blood earlier), but too often he makes them objects for sight gags. One is shot in the chest by a flare gun; for some reason that ignites its head. Another is force fed from a fire extinguisher until its eyes pop out of its head.