No it is not 100% "safe", github can go down while you need to checkout the source, the author can delete the repository (or do some disrupting change to it) ecc. ecc.
With pip you can specify a revision or a tag together with the repository link
eg.
git+git://github.com/misterx/projectname.git@840d25bb9db9fbc801b9
this will checkout the revision 840d25bb9db9fbc801b9 no matter of the new versions so you do not end with unknown newer (broken) code.
What I normally do is to clone the project (unless I want to keep in sync with trunk) to my github account or somewhere else.