I actually wrote a similar blog post about switching to this stack (haodong.io/migrated-blog-to-gatsby...). I've changed my stack quite a few times in the past few years (Ghost blog, Hexo, Hugo, Github Pages and now Gatsby), then I settled on this stack.
The biggest reason for not picking Netlify CMS (which is still an awesome tool) imo is the lack of features. It offered very few customisations e.g. didn't even have a "draft" and "published" status for the blog post so anything you write you have to publish. Of course, this was when I tried it out a few months back maybe it is different now. With Contentful, there are tones of customisations: extensions, dynamic forms fields and etc. I also love the fact you can query Contentful through graphql.
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