What does GitHub do well? Github helps me depend every time less and less to local git management tools, such as Git Bash. Having a user interface to do things like cloning repositories, forking and commiting is very handy when you want to make your git experienc eportable and collaborative.
What does GitHub not do well? Git merge seems a little complicated in github, it seems to me it is a little easier to compare and do code reviews by using an interface similar to that found in GitLab.
What do you wish you had known about GitHub before using? I wish I knew private repositories would be free after Microsoft acquiring GitHub. I would've used github as my primary git driver.