"Safe" means security, user safety, and dealing with the new range of unknown that AR offers.
MIT is looking at practical security issues, like malicious software, physical risks to users, corruption of information, and all the other wonderful things we've come to expect from any kind of software. (The MIT article is much too short; it does address all relevant points, but really needed more space.) As MIT points out, one of the most critical issues with AR is its immersive nature. Users are almost literally glued to AR in ways other technologies simply cannot approach