I also have a new ES ED127 FCD100 OTA, but rather than the ES flattener/reducer, I have a Hotech SCA FFT58-2" flattener, and an Astro-Tech ATR8 reducer/flattener.
The Hotech requires both provided extension rings (each is 37.5 mm), giving 75 mm of extension between the hexagonal draw-tube and the adapter ring. With the sensor of my ASI 1600MM 55 mm behind the back ring of the Hotech, I hit focus at about mid-way on the focuser. Using the Astro-Tech with similar back-focus and with both extension rings, the draw-tube is pretty close to minimally extended at focus. Obviously, I'm not using the diagonal.
The focuser on mine was also flopping around on arrival with the grub screws almost right out of the threaded holes in the rotation ring, and the rotation locking screw was over-tightened and boring into the inner ring metal. The draw-tube also had almost a millimeter of lateral free play within the focuser until its adjustment grub screws were correctly tightened and the focuser and draw-tube tube aligned. Coarse focusing movement was rough and gritty. Once the focuser was stripped down, cleaned and reassembled correctly, it now works nicely with no free play and no image shift, and no drift with the camera and filter wheel attached. The movement is now smooth and positive with a nice feel to both fine and coarse adjustment.
More concerning is the fine dust and several very tiny bits of shiny metal swarf visible between the objective elements, and tiny flakes of the black paint on the diagonal mirror. No effect on the image, but not what I'd hope to find on a brand-new scope at this price.
On the positive side, once the focuser was cleaned and aligned, the OTA and Hotech flattener give nice round, sharply-focused stars to the edges of the field on my ASI 1600MM. I don't see visible vignetting, but I haven't looked hard for it. The weather has not yet been good enough to test things out much further. It came with an ample free gift of cloud and rain, of course - and even this very limited testing has been done dodging between the showers, clouds and heavy dew.