Six months ago I moved DesParoz.com over to SquareSpace, specifically SquareSpace 6. Today I am moving it back to a self-hosted Wordpress installation.
I really like SquareSpace, and continue to run 3 important sites  on SquareSpace, and continually heartily recommend SquareSpace as a great option for businesses and organisations who need a fully featured website. But for a site that is primarily a blog, such as DesParoz.com, I was frustrated with the process for publishing an article.
With Wordpress, I can write and publish directly from Byword - both Byword for iOS and Byword for Mac. On SquareSpace, I would have to write in the editor, then copy the HTML, then paste it into the SquareSpace website (or app). It was simply too many steps for my liking.
Frictionless workflows are important, so after considering this change for a while I made the decision to switch back. Frankly my life is likely to be even more on the go in the forseeable future. The ability to write and post on the go is critical. So while I lose some of the powerful features of SquareSpace, I gain the flexibility of Wordpress and Byword.
My workflows are based on the front-end apps I choose to use on a daily basis. Backend systems, whilst important, should be chosen based on how well they support workflows and apps.