I consider myself a decently knowledgeable person in American history from 30’s to the 80’s. As I watch these mini docs, some stuff I can recognize, others I am learning about for the first time, and some have changed my perspective as to how things actually happened. So far, I believe 100% of it, but I know that Oliver Stone is well-known for putting a lot if opinionated material in his films. Although I feel like this series would be different given its a documentary-based series, I still feel like some stuff has the potential to be wrong.
Is there a specific claim that you are curious about? An entire documentary series is a bit broad and unspecific.
These previous answers may be of use to you. This is not intended to discourage questions -- any follow-on questions, information, or debate is welcome.
"How Accurate Is Oliver Stone's Untold History of the United States?", the one from 1 year ago, https://www.reddit.com/r/AskHistorians/comments/5fg3z2/how_accurate_is_oliver_stones_untold_history_of/ : actually has some meat and discussion, on William Wallace and the first uses of the atomic bomb. /u/restricteddata (of course) provides a good evaluation. There are a lot of other comments about other aspects, but not touching on Oliver Stone.
Edit: the Original Poster clarified that that's indeed the focus of their question. There's an FAQ section at, I kid you not, "The atomic bombs, aka questions /u/restricteddata has answered", at https://www.reddit.com/r/AskHistorians/wiki/faq/militaryhistory/wwii/usa#wiki_the_atomic_bombs.2C_aka_questions_.2Fu.2Frestricteddata_has_answered . Lots of meaty discussion, including "Did the US have to nuke Japan in WWII?".
The FAQ section is at https://www.reddit.com/r/AskHistorians/wiki/historians_views#wiki_historians.27_views_of_oliver_stone.27s_.27untold_history_of_the_united_states.27.3F . It doesn't have a lot of meat there, unfortunately, and the one above isn't there.
"How is the historical community reponding to The Untold History of the United States", http://www.reddit.com/r/AskHistorians/comments/13j866/how_is_the_historical_community_reponding_to_the/ : /u/Samuel_Gompers , /u/Dzukian , mostly talking about William Wallace &c.
"Saw a video of Oliver Stone's "Untold American History" and am wondering whether I should watch it?", https://www.reddit.com/r/AskHistorians/comments/14r9ui/saw_a_video_of_oliver_stones_untold_american/ : LordKettering , but there's not much discussion here, more like assertions.
"I have started watching "Oliver Stone's Untold History Of The United States". I am not a historian. What should I know?", https://www.reddit.com/r/AskHistorians/comments/14ypvc/i_have_started_watching_oliver_stones_untold/ : ShroudofTuring , but again, thin on examples and discussion.
"How accurate is Oliver Stone's Untold History of the United States?", the one from 5 years ago ... this one isn't even worth it.