The problem I had with Django wasn't that it didn't scale in the sense of "heaviness", but rather that the moment you wanted to do something that was outside the typical expectations Django had about a webapp's behavior, it punished you for it.

A more extreme example might be the agony you go through in attempting to get Rails to do something non-standard.

The client testing in Flask is a GODSEND. I love it dearly. It even supports the session context properly!