In epistolary short story science fiction, the pacing is often different. It can be more episodic as each letter or entry might be a different 'episode'. Also, the characters are developed in a unique way. Their personalities are shown through their written words. And the science - fiction elements, like futuristic technology or alien species, are described in a way that fits within the epistolary framework, usually through the eyes of the writer of the letters or documents.
Epistolary short story science fiction often has a strong sense of intimacy. Because it's like reading someone's private thoughts or communications. The world - building in this type of story is also interesting. It has to be done gradually through the letters. The science - fiction concepts are introduced bit by bit. For instance, if there's a new form of energy in the story, it might be described in a letter as a discovery. This type of story also allows for a lot of speculation and philosophical musings within the science - fiction context.