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Amazon Storywriter Reviews

Cloud storage is a plus

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It doesn't appear to be much publicized, but this uses Amazon's very robust Simple Storage Service (aka AWS S3) as the storage backend. As such, it gives the benefits of that platform, transparently.
Export formats are limited, but appropriate for the target audience (Fountain and FDX, pdf formats). There are numerous converters (and copy-paste options) if those formats don't work for you.