Stalone himself said about _First Blood II_ it's more like a fantasy. The script was supposed to be an attempt to right a wrong and to show how it should have been (took this from an interview on a DVD). He also said it didn't work out.
The dumbest move was to bring the Russians in. That doesn't work for me because at that moment it became a typical east vs west cold war confrontation and the whole Vietnam thing took a backseat. Yes, I know the Russians were involved behind the scenes historically. Still, this shouldn't be part of the movie.
Something about Stalones acting that was oftentimes portrayed as one dimensional and Rambo himself being kind of stupid. After Stalones explanation that John Rambo is basically an adolescent in a man´s body who stopped developement when he joined at the age of 19 I think all this was intensional.
Beside that it is still very watchable. What really works in favor of the movie is that all you see on screen was done in pratical shooting. Which ages so much better than anything CGI. Even better in HD.
But it stays a good step behind part one.