Alright, sure.

GitHub is owned by Microsoft. Microsoft has a long history of opposing open-source, like its founder rallying against it before the term even existed, or its former CEO calling it a cancer.

Microsoft is also quite prominently known for a strategy of embracing open standards they don't like, to kill them off. Particularly, they might be looking to extend and displace Git.

Microsoft also violates the licenses of the open-source projects on GitHub.
As someone publishing open-source software, I do not put the license for fun in there. If I know for a fact that it's being violated, I will not publish my code. For the same reasons, I also don't contribute to projects on GitHub.

And last but not least, Microsoft also has a history of violating privacy laws.
Particularly, if your project advertises itself as privacy-centric, like this browser does, then this excludes your users from reporting bugs, discussing the direction of the project or contributing.