My complaint with Django is/was that it's fantastic for building brand new apps starting from scratch, but less pleasure to integrate with existing databases. The last time I tried to add Django models to a DB we were already using, there was an impedance mismatch which made it hard to fully model, and I gave up trying to get the admin to work well with it. The ORM and admin are 2 of Django's biggest draws, perhaps the biggest. Without them, it's not so pleasant.

That's when I first came to love Flask. SQLAlchemy will let you model just about anything that looks vaguely database-like, and Flask doesn't really care what ORM (if any) you use.

TL;DR Django's opinionated. If those opinions match what you're trying to do and you can stay on the golden path, it's freaking great! Once you get off in the weeds, it quickly becomes your enemy.