Most of the prior projects I've seen that used public github data for anything along these lines suffer from the same flaw - Many coders' work is private. You can't see it to include in the system, so what you are really ranking is people's public code. And for many devs, that is their experiments, not their best work.
this is true. we could request private commit permissions but I was afraid that would scare people away from signing up. If there is demand for it we could enable it and it would certainly paint a more complete picture - with the obvious caveat that we'd need a large number of users to sign up and grant those permissions.