While Django is mature, I'm using it right now in fact, I find that I spend a hell of a lot of time trying to figure out why Django is doing a specific thing or how to convince it to do the exact thing I want.
I'm convinced, after a few years of Django work and a year or so of Flask (or just werkzeug libraries), that Django is the wrong level of complexity for a web framework.
For every minute I've saved by having django handle the edge case or having some great feature built in I feel like I've lost a few minutes to fighting against the framework when my use case doesn't fit into the expected usage and I'm fighting against a fairly rigid framework.
This feeling is what I think the OP is trying to get at with the "excessive complexity" measurement, although using a single metric like this is probably not the answer. It does fit decently well with my impressions of the frameworks listed (among the ones I've used anyway) but this is a messy and subjective problem.
I've found myself firmly in the "libraries > frameworks" camp because of this issue so I'm certainly biased towards liking this metric, even with it's faults.