In addition to the flaws mentioned by the other commenters (whole word only, no regexes), Github will only search the master branch of a repo, which makes it useless for my two most common tasks "where did this code used to be used" and "does this unmerged PR manage to remove all of the references to this code in its refactor (incl. in comments/etc)"

EDIT: Unfortunately, it seems like search doesn't work at all on this site, which is a shame. (at least, when I tried to do it on https://github1s.com/rails/rails it failed)