Meanwhile Nixon, who in the film is shown as doing these interviews for the sole purpose of rebuilding his reputation, suddenly capitulates? Why? It's utterly arbitrary and therefore unsatisfying; you cannot believe that either of these characters would have made the transition that they did. Even as a film, without reference to the history, it does not work.
As history, it's much worse. There are a few important things left out of the film. First and foremost, Nixon didn't make that admission. The transcript of the interview is shamelessly edited to almost completely reverse what Nixon actually did say, as several commentators have pointed out.
Secondly, the film is set up as a gladiatorial battle, in which only one of the two combatants can come out victorious. This is not what happened. Nixon was not only paid for the interview, he was given a sizeable cut of the profits. So it was in his considerable financial interest to make them successful. So by offering up some juicy Watergate tidbits, he gave Frost the victory he needed and ensured some financial security for himself. It was good for both of them and Peter Morgan in fact said he could have written it to have Nixon "win" with as much historical justification.
I think the thing that disturbed me most, though, was that Langella portrayed Nixon as in many ways a likeable man. He wasn't. He was nasty and vengeful and probably as close to evil as any American President has been, and to portray him as a sympathetic character is not only dishonest but disturbing.