Like a lot of semi-historical figures, Robin Hood exists in a nether realm between reality and fantasy. There is a real debate about whether there was actually a man who existed named Robin Hood or if he was always just folklore. More importantly, a Robin Hood story can be grounded in historical events or pushed towards pure symbolism depending on the story teller's whims.
At first, it seems like Robin and Marian is going to be on the more mythological end of the spectrum. The first third of the movie feels like it's an absurdist comedy, where Robin is a beleaguered everyman stuck in a world that's gone mad. In one early scene Robin is ordered by the King to besiege a castle that looks like it has been abandoned so they can steal a golden statue that Robin is pretty sure has already been stolen, and also, probably isn't actually made out of gold. After he refuses to comply with this nonsensical order, the King besieges the castle anyway, kills a bunch of children (off screen), and discovers that there was no statue, and also, the statue wasn't gold. It's like an Emperor-has-no-clothes situation, except darker.
Even though Robin ended up being correct about the statue, he is still put in a jail cell to await certain death at the King's hand. Soon enough the clever Robin notices that a stone near the top of the cell is loose, so he stands on Little John's shoulders and tries to work it free, struggling hard all through the night. He gets it completely free at the exact second that the guard reappears to summon them to the impromptu throne room. There's an emotional truth to these early scenes, but they don't feel like things that would really have happened; they're clearly just metaphors trying to explore the futility of life.
Unfortunately, the opening tone of the movie is misleading, because the majority of the movie tends towards a more grounded vision. For example, there's a scene in the film's middle where Maid Marian, who has never once left the area around Sherwood forest, asks Robin what the Crusades were like. Robin answers her by telling a story about a siege where after the Christians had finally taken the city they slaughtered everyone and then slit their guts open to make sure that they hadn't swallowed any jewels. On some level, that story shares a brutality with the siege the movie opens up with - but there's no black humor in it. It seems distressingly plausible in a way that the film's more fanciful opening does not.
The rest of the movie is also in that weary vein. Robin never wrote Marian when he was abroad because he cannot read or write; his anger at the Sheriff of Nottingham doesn't come out of a noble love for the people, it comes from a sense of bitterness that his years of sacrifice for the crown aren't respected in his own home; it even carries over into the fight scenes, which are a far cry from the swashbuckling scenes you often see in Robin Hood movies. When Robin faces the Sheriff of Nottingham at the end, they do so in bulky army and with heavy swords. They sweat. They grow short of breath. When they are cut they bleed, and the more they bleed the harder it is for them to fight. Before the fight is over neither of them can stand upright and lifting their swords takes considerable effort. By that point in the movie I doubt any viewer would be expecting a happy ending, but the fight's obvious brutality makes it clear that any victory either of them could achieve would surely be Pyrrhic.
That gritty tone is good - in fact, it's kind of a refreshing change, given how formulaic the mythologically minded Robin Hood movies tend to be. The film's more comic scenes are also good - I'm cynical enough to respond to their bite. I can even see how someone would think those two tones would fit together. After all, the Robin Hood myth is big enough to encompass both a real person and a larger than life figure. But while "Robin Hood" is big enough to be both a man and a metaphor, sadly, Robin and Marian is not.