The Rambo series fell apart when Stallone decided to make it a series.
I have to admit, I thought the original sequel was entertaining and gave Rambo something of a 'redemption': going back to Vietnam, saving some POWs...and you know that he ended up back in prison after that "mission accomplished" bit.
At the same time, if that sequel had never happened (or had bombed) Stallone MIGHT have dialed down some of the cartoonish stuff thereafter, both in the standalone films (Cobra) and the Rocky series.