On piece of data that is easy to lose, with any version control system I've worked with so far, is uncommitted data. And that's also the only data I've lost with git so far, after using it for several years. (And yes, it was my own stupidity, saying 'git checkout .' and only noticing later that there was something I wanted to keep).

The advise to take a backup doesn't hurt, and might be helpful if restoring the original state is more effort than doing it with git operations.