I have been using the holder for a few months now and have had different experiences on different cars.
The magnetic connection basically holds quite reliably. Even with a protective cover on the smartphone.
However, the reliability of the attachment of the holder to the air vents can vary from car to car. The prerequisite is that the plastic bars of the air vents have sufficient distance from each other and are not too deep. It is important that the holder can be pushed in so far that the small beak reaches behind the plastic bar and thus cannot slip off. If this is the case, I attach the holder to the lowest bar so that it rests on the frame of the ventilation outlet and hardly wobbles any more. Then, in my experience, there is no longer any reason to worry.