I thought the accents were pretty close to be honest. The scenery shots were stunning, I would bet that at least 75% of the people that live downstate have no idea how beautiful the rest of NY is. When Tilly bought the boston creme and kruller from the donut shop, that definitely is a regional thing.
I also find it funny that there was an issue with Tilly's s e x scenes, it let the viewer get an understanding of her mental state and character. The real life Joyce Mitchell had a miserable childhood, she also is a serial liar.
Thought the best episode was 6 when we got a look at why Matt and Sweat ended up in jail. It left me stunned. The fade away shot of Tilly at the end was fitting.
Vulture has a side by side comparison for each main character and their real life counterpart with a brief synopsis for each.
How Escape at Dannemora Actors Compare to Their Real-life Counterparts
Edit: Patricia Arquette gained 40 pounds for the role.