These are exciting news, especially about Draft.js editor integration! I've built multiple projects with Wagtail CMS, and it's always been a great joy to work with the code as a developer. Clients also like it for its sleek and uncluttered admin UI.
Played with Wagtail and the first thing I wanted to find out is how to customize the wysiwyg editor. Is there any API to add / remove buttons, markup, etc? Is it possible or we're stuck with what's offered?
The same can be applied to Rails (let's compare to it) too:
1) You're at the mercy of the gem developers and Rails changes versions faster than Drupal, breaking backwards compability.
2) Number of gems also does have an influence on Rails app performance, at least on an app's startup time.
Keep it up!