When I started upgrading my camera to a heavier model and adding accessories to it, I could notice some chromatic aberration on the stars. When the focuser was fully extended, the imaging quality was slightly affected, due to a slight bending of the optical trein's main axis.
I initially thought this effect was due to a sensor tilting, which I tried to solve with a tilter. But it was pointless, as the issue was not the camera related tilt, nor any filter in between or the reducer. My chromatic aberration was dependent on the extension of the focuser. Not really a straightforward troubleshooting, which has given me some headaches.
Though, this is not too much of a problem as this issue gets mostly corrected by re-allining the RGB channels in post production. However, better not to have this issue at all. I will soon upgrade to the WIFD version, as it's meant to solve this structural issue for good. Please note that this issue affects pretty much all refractors with extensible focuser, so William Optics seems to be actually the first brand to have solved this problem with their patented WIFD technology.

Lens Type Triplet Apochromatic Prime Lens
Focal Length 478 mm
Diameter 81 mm
Aperture f/5.9
Image circle 45 mm
Focuser 2.5" R&P Compatible with EAF
Adapter M63 Rotator 2" → 1.25" Visual Back
Flattener FLAT 6AIII 0.8x FLAT GT for f/4.7
Camera Format Full Frame APS-C M4/3
Tube Length 390 mm - 460 mm
OTA Weight 3.9 kg / 8.6 lbs