Whenever an author writes a frame story that accompanies the main narrative there are certainly advantages and disadvantages as is true of Mary Shelley's Frankenstein.
In writing a novel that raises the issue of the dangers of science with reference to the writings of Erasmus Darwin, who prefigured the discoveries of his grandson Charles, and exploration into dangerous realms such as the Artic in her novel, Mary Shelley tempered the shocking impact of Victor Frankenstein's creation of a living being with the character of Walton, who acts as a counterpoint to Victor in some ways and assuages some of the criticisms that the narrative was "an uncouth story...leading to no conclusion either moral or philosophical."
With another narrator in the epistles that frame the main narrative, there is a voice of more normalcy to which readers can relate. In addition, Walton's sympathy toward Victor Frankenstein at the end of the novel re-humanizes Victor. Moreover, with the inclusion of the character of Walton, readers more readily identify with the themes of isolation and alienation. Walton's final decision to turn back after listening to his crew also mitigates the harshness of Victor Frankenstein's story.
Whenever there are two narratives, that at times seem disparate, within a novel, there is always a threat to the book's coherence. For, the jump from Walton's loving letters to his sister and his more realistic idealism are in sharp contrast to Victor's hubris and his divorcing himself from his loving family to pursue his maniacal aspirations
Often verisimilitude is compromised by two separate narratives. For instance, in the last chapters of Frankenstein, it seems rather too convenient for Walton to rescue Victor from the icy waters, rather like the artificial use of deus ex machina.
In addition, Shelley strains the reader's credulity with two fantastic ventures that somehow meet each other in the "futility of freedom" and become friends. Walton, the man of science, readily believes Victor's tale. He writes to his sister about Victor,
His tale is connected, and told with an appearance of the simplest truth....My thoughts, and every feeling of my soul, have been drunk up by the interest for my guest, which this tale, and his own elevated and gentle manners, have created.
The sympathy of Walton and the opportunity for Victor to relieve his soul on the ship seem a bit too convenient a way for Shelley to make Victor Frankenstein, who has sacrificed his family and friends in his hubristic drive, a more human character.