“By using the REST API and heavy JS, CCF can do things like live form previews which Gravity Forms and Ninja Forms can’t do"

Funny, last I checked the Gravity Forms form editor was JS based and didn't reload the page as you add and edit fields to the form you are building. The form editor also provides a live preview of your form as you are building it.

When you embed a form on the front end you can also do so using AJAX so multi-page forms don't require a page reload as the user moves from page to page in the form. But the forms also work without AJAX and with page reloads for users with JS disabled.

Just because something is built using the WordPress API doesn't mean it's better than something that was not.

Word of advice: Focus on your own product and what you are doing instead of making false claims about your competition and throwing around buzzwords to hype up your product.