I caught Rambo on DVD this evening. A few thoughts:
The Christian missionaries were poorly drawn. Let's start with the easy one: what was the girl Sarah doing on this trip? These were supposed to be medical missionaries, but the most constructive thing she did during the whole movie was "hand me a bandage." For the rest, she was either wandering around aimlessly, or running in terror. If Michael, the team leader, had really made four trips into Burma, then he would have known perfectly well that it was no place for a woman without skills.
More importantly, Evangelicals don't talk the way these people talk. Their Christianity has been watered down to the most New-Agey claptrap imaginable. At one point, Sarah asks Rambo if he has "lost his faith in people." No properly educated Christian has faith in people. (It could be that I haven't sufficiently kept pace with the decline of Evangelicalism.) Also, Michael is shown objecting to Rambo's early use of violence to protect them from Burmese pirates, saying "it's never okay to take a life." Granted, there are Christians that actually believe this, but they tend not to be Evangelicals from Colorado.
The mercenaries are poorly drawn. Lewis, the merc leader, is "old school SAS", but spends his first 15 minutes of screen time whining about being in the jungle. Somewhat implausibly, he knows Rambo works locally catching snakes, but evidently doesn't know that Rambo is former SF, and rags on him for no apparent reason. This appears to be a staple of action movies: no matter what their obvious physicality, the heros get picked on by people manifestly weaker than themselves.
More broadly, what is this bunch of guys with these kinds of skills doing freelancing in Thailand? For missionaries? They should be working for Blackwater; or if their records are dirty, then providing security to drug smuggling.
The effects were so over the top, they reminded me of the Rambo parody in the movie UHF: sniper bullets decapitating people, bodies bursting like water balloons, that kind of thing. Couldn't they have done this right?
The Burmese military is shown to be really bad, but completely without context. Nary a word is uttered about the Burmese Muslim persecution of Christians; we're supposed to believe that the army randomly uses civilians for target practice.
In its favor, Stallone is half-way plausible as a burned out 'Nam vet, and the combat sequences are marginally more realistic than those of the previous films, but that's not saying much. If I was still 14 years old, I would probably not be complaining. But it would have been easy to make this a serious film about a serious issue, and what we got instead was a cartoon version of reality.