I read the novel whenever it came out, and, though I tend to like Maggie O'Farrell's earlier work, I thought this was tiresomely 'manic pixie dream witch' stuff, all Agnes being earthy and wise and woodland-creature-y as distinct from the townspeople. Also, the prose took so much time over every single Elizabethan detail (women's caps, glovestretchers, woven bee skeps etc) that reading it (and I am someone who likes a slow, non-plotty novel) was like wading through jam.
The film I found uninvolving, despite liking Chloe Zhao's other work, and admiring Jessie Buckley (who was astonishing in Women Talking). I think that without MOF's prose, it just felt like a standard costume drama, with a good cast, but one in which I never suspended my disbelief at all.
And it just got some stuff wrong. I don't mean the front crawl, the Harris hawk or the echinacea, or people saying 'OK'. The Elizabethans just weren't us. They believed in things we don't believe in. Their religious world was in flux between Catholicism and Protestantism, with key things like purgatory being disinvented in the shift. Their lives were different. Many brides were pregnant at the altar, as the marriage contract was regarded by many as license to have sex. Between one third and half of all children died before they hit puberty, so Hamnet's death was not singular, it was a (to us) shockingly ordinary thing.
Shakespeare's players are highly unlikely to have been fluffing their lines that much in a rehearsal close to the first performance -- it was a demanding, highly skilled role, they needed astonishing memories as they had to have dozens of parts in their repertoire at any one time, and the acting style was loud and fast, even in a play we think of as introspective. It had to be, they were competing with bear baiting and public executions. They weren't methody. The film's groundlings were way too clean, and well-behaved (no one would have been shushing Agnes, they'd have been shouting 'Get on with it, love!' at Hamlet!)
And the stuff that the film had to do to make Hamnet's death relevant to Hamlet felt like a bit of a botch job.
Shakespeare was writing a stream of comedies at the time of Hamnet's death (and some people have suggested that all those separated twins, who often believe the other is dead, like Olivia in Twelfth Night, are in some sense about Hamnet and Judith).
Hamlet is from several years later, and the lead was played by the superstar of his day, Richard Burbage, who was in his 30s, not a cute young blond who might remind a grieving mother of her dead boy grown up. (And the backcloth wouldn't have been a wood!)
The amount of chopping around the film has to do to make Hamlet, a revenge tragedy with an early existentialist stuck into it, anything that might plausibly allow it to be a play that is about a father grieving a lost son is a bit clumsy. That scene with the Ghost and Hamlet is mostly the Ghost talking about his horrible torments in Purgatory and telling Hamlet to revenge his murder, but not to hurt his mother, even though she's shagging his murderer!
Jessie Buckley might well say 'How is this about my son?' Because it isn't!