Don't listen to the critics. The viewing length is long, but it's a great film. It obviously sets up the rest of the story that is intended to appear in future chapters. The initial story covers several storylines and characters, which requires the viewer to pay attention. I loved the film and can't wait to get into the next chapter. As others have stated, Costner could have released these as a miniseries or a single-season television show like Sheridan did with Yellowstone 1883.
I saw glimpses of the dynamics that many pioneers likely faced during westward expansion. Costner even gives us a glimpse into how the Native American's carried opposing views about how to coexist with the incoming pioneers. I'm curious to see how these stories play out in future chapters.