Hey Ed,
agreed. At least what in any case I expect is, neither the QCW Frame nor a Kreg table can prevent that they twist according to the surface you put them on. And a twisted machine can only produce twisted workpieces as result.
But I would not think that a table with diagonals, which of course is a much, much more stiffier and stable then, can prevent twisting according to the surface you put them on. The test is: Put a coin under one feet. If it is that stiff and stable as you think, the table should wobble then. But It won’t, because to have a machine base that really does not twist according to the uneven ground, it would need to be welded of half-inch thick steel profiles and annealed for stress relief like shown here. And even such machines have something like the Any Level Surfacing System, as you can see here on this heavy machine made of welded steel.
So if you have a twisted machine, it can only produce twisted workpieces as result, and removing twist (=ensuring coplanarity) is mandatory before starting to mill something. And I don’t want to do this crawling around on the floor on height-adjustable feet or casters, but standing upright, by adjusting the Any Surface Leveling System at the height of the frame.