* The developer mode is poorly supported. For example: you can't copy your site if it is in developer mode (which means you can't setup a staging environment and everything you push to your site will be live instantly). The local development server is also very buggy and inconsistent with the production behavior. It feels like something that they developed early on in their road map and haven't spent much time on since then. If I could go back and do this over again, I'd pick an existing theme and just customize it instead of building out a whole theme in developer mode.
* The site is S-L-O-W with tons of bundled assets that it injects into your theme. I have no idea what any of it is doing, but it definitely increases page load.
That said, if you find yourself wanting a solid CMS that has the full breadth of WordPress like features but don't want to deal with the hosting, SquareSpace is a solid choice.