Presumably the fact that they take down repositories that disagree with their politics (or the politics of other governments). They also don't appear to put up any fight against DMCA's, and they mandate that all users must run proprietary JavaScript. LibreJS makes the site mostly work though, so it's not that bad. The biggest problem is their policies, which are not pro-free-software (no matter what they might say).
I am receptive to this kind of argument, I am honestly curious for specifics.