Laurent,
I can see your point but in the case of the SX2 where the nut would be very close to the gibs I don't think this will apply, there have been literally thousands of CNC mills built or converted of the Bridgeport type where the nut block is secured to the front of the quill which would have the same effect but in a front to back application instaed of a side to side.

In fact my big Beaver mill has exactly the same layout on the head and the Y movement is even worse as the ball screw hangs off one side, only the X has a central screw.Click image for larger version. 

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