Much like how Desperado was a weak follow-up to El Mariachi, Once Upon a Time in Mexico continues the trend of being a mediocre action film that pales in comparison to the hilarious film that kickstarted this trilogy of films.
The most glaring issue with this film is the sheer amount of things that are taking place. El Mariachi was a simple case of mistaken identity that culminated in a crime story. Desperado was a revenge story that, while messy in terms of plot, still had a simple enough premise and a moderately satisfying conclusion. In Once Upon a Time in Mexico, the story is based around a presidential assassination the Mariachi happens to get himself wrapped up in... but there's also his own personal quest for revenge, [spoiler] yeah, he's lost another girlfriend, and this subplot feels incredibly forced into the film, [/spoiler] there's a plot in which **Johnny Depp** is trying to assassinate a character played by **Eva Mendes**, and the Mariachi has these two prat friends who are trying to rescue the president simply because of the reward in the end. The rest of the film is honestly too complex for me to explain, and it'll make your head spin - especially compared to El Mariachi and even Desperado.
The other main problem is that the Mariachi is barely in the film. The main story of the film really focuses on the politics centred around the assassination - with the real protagonist of the film being Sands, a CIA agent played by Johnny Depp. And to be fair to Depp, his performance is much more interesting than the portrayal of the Mariachi over the last two films. He's definitely the highlight of this film... but that's the problem. Mariachi is objectively the protagonist of the film, but he's also the one with the least control over the outcome of the plot, and that should never be the case with the main protagonist.
The film isn't all doom and gloom, however. As I said before, Johnny Depp is easily the best thing in the film, and he does give a pretty solid performance, as does **Willem Dafoe**. Even **Antonio Banderas**, despite (or perhaps because of) having less screentime, gives an improved performance as the Mariachi. In fairness, I don't have too much to criticise in terms of acting for this film - it's all reasonably decent and while not outstanding, it does the job.
The action is also fun, minus one town shootout with abysmal shaky-cam and rapid editing. The following motorbike chase is enjoyable, and there's a very fun gore effect towards the end of the film that honestly caught me off guard a bit. There are things to appreciate in this film, but it's unfortunately a little bit clustered for its own good.
Overall, I'd consider El Mariachi to be the undisputed best film in the Mexico trilogy, with Desperado and Once Upon a Time in Mexico being on roughly the same level - although choosing between the two latter films, I'd go with Desperado purely for being less clustered and more enjoyable.