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.
If you don’t have a design muscle in your body, you might experience some difficulties making a Squarespace site look good (due to the platform’s reliance on good stock imagery to be part of your site’s final look). Also, Squarespace offers a good range of features from the get-go, but above that, there’s not much you can do when there’s a feature missing. Just like with Wix, if you want a site that can grow alongside your business, this might not be quite the solution you’re looking for.