Didn't have any interesting characters
All of the Rambo movies, aside from this one, had at least one interesting character other than Rambo himself. The first movie had Trautman and Teasle, the second movie had Trautman and Murdock, the third movie had Trautman and... well, just Trautman I guess. The fourth movie had the mercenaries; Lewis, School Boy, and Reese in particular.
This movie had what, some generic Mexican villains? None of the characters in this movie were even remotely interesting or otherwise entertaining.