TL;DR: Mostly accurate.
The HBO mini-series "Chernobyl" is now television's highest-rated series. On June 5, 2019, it jumped into the No. 1 spot on IMDb's all-time TV rankings, with a 9.7-star rating (out of 10).
That puts it ahead of AMC's "Breaking Bad" (9.5), BBC's "Planet Earth II" (9.5), HBO's "Band of Brothers" (9.5), the original "Planet Earth" (9.4), HBO's "Game of Thrones" (9.3) and HBO's "The Wire" (9.3).
Ulana Khomyuk wasn't real and Dyatlov wasn't that bad
First, the character of Ulana Khomyuk, brilliantly played by Emily Watson, was not a real person.
At the end of Episode 5 of "Chernobyl", a picture is displayed showing a bus full of people with overlaying text saying, "(Valery) Legasov was aided by dozens of scientists who worked tirelessly alongside him at Chernobyl.
The character of Ulana Knomyuk was created to represent them all and honor their dedication and service to truth and humanity."
The undoubted villain of "Chernobyl" is Deputy Chief Engineer, Anatoly Dyatlov. It was he who insisted that the test of the turbine's ability to supply electricity to coolant pumps in the event of a power failure continue despite instability in the core.
However, scientists have concluded that, "poor quality of operating procedures and instructions, and their conflicting characters put a heavy burden on the operating crew, including the Chief Engineer."
n spite of that Dyatlov, along with Plant Director Viktor Bryukhanov, and Chief Engineer, Nikolai Fomin, were sentenced to 10 years in a labor camp for "gross violation of safety regulations."
Bryukhanov was even sentenced to an additional five years in prison for abuse of power, with that sentence running concurrently with his 10-year term.
Rest at RL, including a video containing spoilers.