Grey lenses are safe by all means if they don't spoil within 1 year and end up needing repair. Of course you can improve your odds by testing the lens thoroughly first before buying. Also, cheaper lenses tend to be less worth it since the amount you save is even less.
But that said, sh1t happens... My photo club in NUS bought a Sigma 50-150 F2.8 a few mths back and within a few weeks the AF died. Good thing for the warranty..