The problem is the adjustment of the 2 bearings. They have to roll really nicely to ensure a stable bed. If it's not, the rolling of the screw will create little changes in the plane angle leading to problems on the final print. To fix this it is possible to use a non metric threaded rod (designed for 3d printers) which is more expensive, or make better linear bearings with less play and better rolling surface. But this will also increase the overall cost of the machine.