I will edit this to add that people who took action at a particular time do get to speak up and bear witness, of course, and their words carry a special weight.
I suspect that The Butler has a certain amount of "great moments" to its storytelling, which might blunt its effect, but I'd have to see it for myself to verify that. No matter how sincere a filmmaker is, it's very hard to make an effective film about a large event or the entire life of a significant person. A movie such as Gandhi probably comes closest. One reason Clint Eastwood's Mandela film, Invictus, is so powerful is that it takes one incident and gives us the texture and humanity of it, and from that incident we can extrapolate the larger picture. The whole idea of how reviled the Springboks were indicates how audacious Mandela was to make the team a symbol of all of South Africa. Remember how entire teams of African athletes boycotted the Montreal Olympics just because New Zealand had played the Springboks? This aspect of Afrikaans culture was an inspired way to approach a portrait of Mandela. But many filmmakers want to take on the whole ball of wax, and they certainly have the right to try. After all, one of them might get it exactly right.