Ridley Scott directed one of the best movies ever made set during the Napoleonic Wars: unfortunately, that movie is not Napoleon but his cinematic debut, The Duellists, forty years ago.
Unsurprisingly, The Duellists had a strong source material (it was based on a novel by Joseph Conrad which it often followed almost verbatim), while Napoleon has an uneven screenplay by David Scarpa.
Even past the age of eighty Sir Ridley can still shoot pretty and energetic pictures but his hits and misses depend on the scripts he picks, and he hasn't always shown the best discernment.
The elephant in the room is the large amount of historical inaccuracies. Even as a history buff I can forgive many of those: cutting or simplifying events for the sake of narrative, or even some overdramatization like the meeting between Napoleon and Wellington (it never happened) or Napoleon being present at Marie Antoinette's execution (he wasn't); however, stuff like Napoleon charging with his troops at Waterloo is absolute cringe, a kid's (or a lout's) idea of history.
Still, the big problems here are characterization and pacing.
The movie is a demythologization (some would say emasculation) of Napoleon. If you want to take this route then fair enough, but the character here fails to be consistent. I can buy a Napoleon who is an egomaniac and an overrated tactician (like in Tolstoy's War and Peace). I do not buy one who is an anxious, insecure, uncharismatic cold fish but also a stern tactical genius and an effective leader of men, one who flees from Egypt because Josephine is unfaithful but is also an unflappable military mastermind.
Phoenix is a great actor and does what he can but the two sides of the character just don't gel with each other. You can't have parodic moments like Napoleon rolling down the stairs during his coup against the Directory, despondently pouting as he waits for the rain to stop at Waterloo or awkwardly climbing on a box to stand face to face with a pharaoh's mummy (with his diminutive stature becoming a not-too-subtle metaphor of his overall mediocrity)... AND THEN have him magnetically charm the French soldiers into obedience after the Elba. This gawky Napoleon would have been shot to pieces there.
The other problem is pacing. A single movie about the whole life of Napoleon is in itself absurd, like making "a movie about World War 2". There is material in Napoleon's life for a VERY dense miniseries (which Steven Spielberg is reportedly planning).
Napoleon's first wife Josephine (Vanessa Kirby) plays a huge role here but I would argue the movie has either too little or way too much of her. This needed to be either focused mostly on Napoleon's personal life or to drastically reduce the (fairly repetitive after a while) moments where Napoleon is obsessed with his wife.
As it is now, it tries to tell - but rushes through - twenty very eventful years of European history and yet devotes more time to Napoleon visiting Josephine after their divorce than to his Russian campaign.
It's like making a D-Day movie which keeps cutting back and forth from the Normandy landings to Hitler spending time with Eva Braun. You can have either The Longest Day or Der Untergang, not both.
Still, it's not worthless. There are some interesting moments and set-pieces and, while Phoenix is saddled with a contradictory character, Kirby at least is excellent.
6/10.
