In one project I worked on, I found this to be very true. We created an enterprise level application, with some constantly shifting specifications.

Flask vs Django was a huge point of contention between the two lead developers. One even quit over Django being chosen.

By the end of the project, we ended up using a ton of django's features. The business functional specifications changed, we did some research, and found out it would be easy to handle via slight extension of some already built-ins to django ex. template loaders,middleware,sites,and signals. It saved us a ton of time. I feel pretty safe saying I would use Flask for small very clearly specified projects, but otherwise I would probably choose Django.