Are modern frame locks "safe"?
They are not any less secure than the old style where they are built into the frame. Even if you were to undo the screws, they would still be a solid ring around your wheel, and would prevent the bike from being ridden as it would come into contact with the seat stays, chain stays, and ground as the wheel went around.  However, in general, these types of locks aren't very secure as they are highly susceptible to the "pick it up and walk/drive away" method of theft. Once the thief has the bike in their own garage, the lock can easily be cut with basic tools like a hacksaw or a grinder/dremel with a cut-off disk