In a web design context probably the biggest advantage is having things like asynchronous code where it makes sense and websockets without having to use another technology stack such as node.js.
Otherwise I simply find we are more productive using Django, it allows less boilerplate code for similar functionality, there is less complexity involved overall.
Disadvantage is maybe for really large projects with many developers, Symfony might be easier to have someone pick up the code since it's more standardized.
Community also goes a long way. Back when I dealt in PHP (+whatever framework) and switched to Django, the average competence of people I interacted with in conversations regarding the language / frameworks jumped up considerably. I like standing on the shoulders of giants.