In my experience flask is great until it isn't - I've seen quite a few projects which outgrew it, and the lack of django's batteries and guard rails left a maintenance nightmare. Not sure if it'll be helpful, but as a result I wrote nanodjango, which lets you write a django site in a single file, then convert it to a full project if you outgrow it.
Crap, now I need to know about competitive Jenga ...