Probably a manufacturing defect in the threads of the material received (thankfully except central!)

The design seems sturdy enough and fluid and silent autofocus bearings made more noise than the sliding carriage!). Can not say what will happen in heavy use. With his feet and wheels design is ingenious and versatile in practice. This material deserves a 4 and even 5 if the truck could be removed easily for screwing its cameras. Otherwise, the easiest way is to screw by turning the cameras on the screw carriage or better perhaps, set on the carriage a solid ball with quick mounting plate.
So why a "3"? . I read in the comments "Several not easily accessible screws on the entire rail to put the slider on a tripod." It is based on this statement I made the purchase. Unfortunately, apart from the no central screw on my copy, it is impossible to determine neither the trépired or other accessories on the other thread (maximum 2-3 turns and impossible to shake). Impossible to set a camera to "remote" mode on more moist than the rail length.
The "usual" use assured, I will not return this slider but try to do these Recut no recalcitrant screw.