I haven't seen it yet, but I went ahead and watched a few Youtubes about it. They weren't really spoilers and all the videos praised the film highly, but the expert consensus is...it's not the true story. The guy- Frank Sheeran- was a bit of a storyteller. He was certainly around and close to Hoffa, but not the killer he claimed to be. And according to cops and the noted ex-gangster, Michael Franzese, not Hoffa's killer.
But, I'll still watch because, I don't expect movies to get everything right and ALL have freakin' raved about the performances.
Now, if you like Scorcese and Dinero (which I just now realized means money in Spanish) check out Mean Streets. A classic!