3. Doesn't Have Third Act Structure
As previously mentioned, the screenplay came before the book was released. This allowed Scorsese and Pileggi to play around with its structure and lead them to intentionally construct the movie without a third act structure — a story structure that is commonly used in many movies.
Scorsese believed the nature of the characters negated him from telling their tales in a conventional way and allowed their creative license to construct his movie around a more episodic manner. The movie does, on occasion, jump from past to present which allows this episodic structure to breath and work within its intense narrative. It's a story told in the manner of six wild horses always threatening to go out of control at any given time, constantly living on the edge. A third-act structure would've stifled those characterisations.