I've been developing with Django since the 0.90 days and, to be honest, I've seen a lot of scalability issues with Django itself once you get big enough.
Those issues all relate to how Django is an all-in-one solution with a lot of modules, which you start dropping one by one as you use different solutions.
For example, Django's cache layer is great, but as you scale you'll want to use Redis clients directly rather than the memcached abstraction.
The ORM is fantastic (and much more intuitive than SQLAlchemy). But as you scale, you'll want to use Postgres' more powerful features without worrying about compatibility with MySQL / SQLite. (Django has made progress in that area with contrib.postgres, but it's still only scratching the surface)
Forms? Templates? Staticfiles? If you're using a JS (Webpack and the like) frontend, forget about this stuff. You're better off not serving the frontend with Django at all, it'll only cause more trouble than is worth.
URL routing... again, nowadays even that is done in Javascript. And doing it in two places at the same time is a pain. Long term, you might drop that too.
What's left? The model layer, and the admin that accompanies it. I wouldn't trade the Django admin for the world but it also has a lot of scalability issues and although it's very declaratively customizable, it's much harder to customize specific models.
The auth layer is also probably something you'll keep for a long time, but it's littered with things you will probably not use such as the whole groups/permissions system, or the adminlogs auditing system (which is super cool but the API is very hard to use outside the Admin).
The problem with Flask is it's missing in the "what's left" parts. And if you use Flask, you're missing out on the entire Django ecosystem of apps that make heavy use of the model layer.
But you dear reader statistically will most likely not hit any of these scalability issues (aside from the frontend ones). So don't feel bad picking Django, it's an excellent framework regardless.