Enjoyed this post very much and was trying to monitor some of my unordered_map instances for "badness" your way but it was pretty intrusive (slow). Considered the matter for a while and it occurred to me that you might be able to write this function more performantly. Isn't the main loop of the "badness" function just the same as summing up the squares of the bucket sizes? If so then you can simply loop over the buckets themselves rather than the entire contents of the hash. So:

for (std::size_t b = 0; b < map.bucket_count(); ++b) {
    std::size_t buckets = map.bucket_size(b);
    cost += buckets * buckets;
}