Epistolary form, telling a story through a series of documents (often letters), is a unique form of storytelling with many benefits and limitations.
One benefit of the epistolary form is that it gives the reader a feeling of watching (perhaps spying) on the correspondence between characters without the storytellers' intent. This voyeurism can be exciting for the reader. It allows the reader to experience the dynamic between characters in a unique and interesting way. Whether or not it is more authentic is open to debate.
There are plenty of drawbacks to the epistolary form. It can be a bit more difficult to experience the motives, objectives, and true essence of the characters in the story without any form of omniscience or direct storytelling. The reader is limited to the documents with which she is presented. Although the reader is, of course, reading regardless, she may grow tired of reading about events rather than experiencing them in words. The difference may seem trivial, but is an essential difference in terms of experience.
