Just imagine Medium shuts down as did Posterous back in the days (which was a similar service). You will not necessarily lose all your content but all the SEO rankings you ever created. To get it right you need to publish on your own domain name and preferably on your root domain (www.yoursite.com/blog) and not a sub-domain (blog.yoursite.com). This has numerous SEO advantages.
Why You Shouldn't Use WordPress! And Why WordPress is Bad
And, finally, don't stress out. These are all tips to make your experience a little easier and avoid potential problems. Keep in mind that it's all about you and your brand. If you look around, you'll find successful sites that have broken almost every one of the rules we've laid out in this guide. (Although no one site has broken all of them, as far as we know.) Google would still be a giant if they'd called it Zugzut.com or Goohoo.com. Good branding makes things easier, but it's still up to you to build it.
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Jekyll is a static-site generator, which lets you create your content as text files that can then be inserted into folders. Once your files are created, Jekyll enables you to build the shell of your site using the Liquid template language. Jekyll stitches your content with the shell, creating a static site that can be readily uploaded to all server types.
I agree with you that Pulse CMS offers an interesting way to go, without databases. Before moving all my sites to WP over the past two years, I had always felt reluctant to use databases: but testing WP had convinced me to go ahead. Although I do not use it at this point (I played a little bit with previous versions), I bought a Pulse CMS license, if only in order to support that interesting project. I do not rule out using it for a site some day, at least experimentally.