Case in point: OpenERP. Serious business software doing serious business things, so of course they can't use any of those other toy ORMs, they have to spend 5000 lines of code writing their own: http://bazaar.launchpad.net/~openerp/openobject-server/trunk...
Hey, it might not be so great, but at least they have tests for all of that code: http://bazaar.launchpad.net/~openerp/openobject-server/trunk...
Er...
I know of other projects in a similar state, but they're smaller teams or solo coders, so I don't know that they're quite as fair game for finger pointing...
My own observation is that bad codebases are the result of bad development practices regardless of framework. In my last job the code quality was so poor (and the management processes so broken) that whether they'd used Django, Flask or roll-your-own would not have made a whit of difference.
At this point I'd prefer to work on a well-managed, professionally coded project written in PHP than one written using <insert-python-framework-du-jour-here> run by idiot managers.
If you think you can do this better than the Django community, this can work out well but I'd say more often than not when I've heard about it people weren't actually able to spend enough time on this as they'd hoped and keep falling further and further behind Django and other frameworks.