Hi Michael
Actually, I haven't used it that much except for a couple simple surfaces. But here is a test for more complex situations.
To be fair, I tried to use these videos as reference to compare freecad and x-nurbs results. ( I just quickly glanced at the curves in the videos, so it's not exactly the same. )
https://www.youtube.com/watch?v=mGbYDIqt_MM
https://www.youtube.com/watch?v=a1Himxz1C-Q

As already known, using freecad's surface filling tool was challenging. You can't just select all the boundaries at once and get the x-nurbs magical results.
I tried to minimize the patch area to make it simpler for freecad. You can see the results in the attached file.

Results are not perfect for sure and I totally understand if you don't want to put your time on it. :)