Is Espresso to React what Flask is to Django, or Sinatra to Rails? I haven't used React, but I far prefer Flask to Django; in many cases you don't need all that extra overhead and complexity that the "full-featured" framework provides.
I wouldn’t compare React to Django and Rails, as in being a “full framework” that is opinionated and has specific ways of doing things. React is still more of just a rendering library than a framework in the traditional sense.