Are modern frame locks "safe"? In the older days I remember that frame locks were soldered onto the frame, but today they seem to be just screwed onto it (example film of installation can be found here).



Doesn't this open for the possibility that someone just unmounts the lock and removes the wheel? (And then takes the wheel to his shop where the lock could be forced and then the wheel combined with another bike that was stolen because the owner only secured the rear wheel to a fixed object)

It doesn't look like the screws becomes covered when the lock is locked.

I assume that the thief would not like to force the lock at site and that the bike frame and front wheel is locked to a fixed object (using a separate lock). The purpose of the lock would then be mainly to counteract someone stealing the rear wheel.
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