It depends on the application. Do you care about how things work inside your database? Do you want to understand how the subsystems of your app (like authentication, security, the API, etc) work? Flask allows you (and forces you) to make your choices explicit. Django makes the decisions for you and hides its workings under layers of abstraction.

I can see using Django to rapidly prototype some kind of CRUD app, but on the other hand, you could just use Wordpress for that. Flask seems like it would be a superior platform for engineering anything non-trivial, especially if you care about how it works.