I'm mainly wary of Squarespace because their entire frontend compiles to YUI which stopped being maintained in August, 2014 (see https://yahooeng.tumblr.com/post/96098168666/important-annou...). You don't write YUI, but that's what the code ends up as when you visit the site. I tried out their developer platform early after they launched it, and trying to be productive with it put my brain in a pretzel.

I feel as though the SS CMS is overpriced for the features you get, and you're locked into their tech stack & templating which I find leave something to be desired. You can use Wordpress as a CMS and use the API to get the data yourself, or just compile the site to static for free with a plugin, not to mention the large number of paid pluggable CMS solutions for developers. The list goes on, but a local development server shouldn't be this big of news for a CMS developer platform.