No, it is not 100% "safe", github can go down, while you need to check the source, the author can delete the repository (or make some changes to it) ecc. ECC.
With pip, you can specify a revision or tag along with a repository link
eg. git + bastard: //github.com/misterx/ projectname.git@840d25bb9db9fbc801b9
this will check the revision of 840d25bb9db9fbc801b9 regardless of new versions so that you don't end up with an unknown new (broken) code.
What I usually do is clone the project (if I don't want to synchronize it with trunk) with my github account or somewhere else.
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