i just started watching under the banner of heaven too, so this is right on the nose.
I read that book and loved it, then got extremely disappointed when I found out that much of the historical side of the book was manufactured narrative without real evidence behind it. Sadly I've found that to be a consistent theme for Krakauer now, who used to be one of my favorite nonfiction authors before I realized that part of the reason he was writing such neat narratives was because he was taking liberty with reality and adding more than could be justified from what he actually knew.
That only goes for the book though - I know nothing about the series. They may have worked harder to factcheck the series than Krakauer did in his book.
And it doesn't change the fact that Mormons (and especially fundamentalist mormons) have to be off their rocker to believe the shyt they believe.