Hi Swissblad. I recently picked up one of these lenses, so I don't have a lot of experience yet.
Roger Cicala at Lensrentals.com tested one of these. It's nowhere near canikon lenses for sharpness until you stop down to f/11. Then it's pretty even.
https://www.lensrentals.com/blog/2013/05/first-look-at-the-rokibowyang-24mm-f3-5-tilt-shift-lens-part-1/
A couple of issues that I ran across are:
1) Using the shift knob when creating a pano. The knob ends up being just under the prism/flash housing and is very difficult to get to.
2) The aperture ring is too close to the focus ring and you can easily change your focus slightly without knowing it.
I also thought the small knobs would be a problem, but so far they seem okay as there is not a lot of torque required to turn them.
Jim