Well, i do not have any experience with cnc, I still building my own and first cnc. But i have build 3d printers and used both flexible and rigid couplers in the past.
The flexible coupler are great to use where the z axis have little weight and the threaded rod is upper and the motor in lower position (in an prusa printer for exemple). The flexible coupler will allow to have a more “fluid” movement and reduce the shock of abrupt movements.
In the case of the cnc you need to have very rigid axis and the use of the flexible coupler can give you the next problems:
– allow an unwanted movement of several mm up and down. When you try to make an precise hole of 10mm for example, pretty sure the flexible Z will go up 1 or 2 mm when you will start to make the hole, was final result the hole will not have an precise 10mm deep.
– if the z axis move a lot, this will up and down movements and create vibrations.
-imprecise deep cuts, sometime it will cut less deep, other it will compensate and cut more.
But you can correct some of this problems by simply have the 5mm motor shaft and 8mm rod all inside the flexible coupler and touching.