She seems charming and talented...and totally wrong for the part. For one thing, apart from her size, she's conventionally pretty. And the big point of Funny Girl is the triumph of a very unconventionally attractive woman (it was callously argued early in the show's history that it was a feel-good story for ugly people--Pauline Kael references this interpretation to refute it in her review of the film version). A very pretty girl who happens to be a little heavy is not doing justice to the story of Fanny Brice. And documentary or no, Funny Girl does mean to tell a version of Brice's story.
Effie in Dreamgirls doesn't have to resemble Florance Ballard. But Fanny Brice in Funny Girl does have to have physical characteristics in common with Fanny Brice.