Hey Tobias,
this was discussed here occasionally. Deflection could come from the fact that on the Onefinity, the Z carriage moves with only one linear bearing per rail:
Image: Onefinity 80 mm spindle mount (for Standard Z-16 Slider). The spindle mount seen here is 2″ (51 mm) high and the linear bearings seen here are 70 mm long, the Z axis travel is 133 mm.
Most CNC mills have two linear bearings per rail, as can be seen here, here (RoverCNC), or here (Sorotec Alu-Line).
Users thought of mounting two 80 mm mounts on top of the other or to simply mount an Universal Z assembly. This would also allow to use the spindle clamps that are usually sold by spindle manufacturers, allowing for other spindle diameters.
Onefinity now offers a Z-20 Heavy Duty Z Slider with 20 mm diameter chrome-plated hollow shafts and 165 mm Z travel that you can retrofit to the Standard Series machines and which replaces the original Z-16 Z Slider with 16 mm shafts, but the spindle mount still runs with only one, however bigger, linear bearing per rail.