It seems to me the freestyle visibility computation is erroneous. I am getting lines where they should not be visible.
This is the scene I am trying to add freestyle lines to:
This is the result including freestyle lines:
As you can see, it seems you can partially see through some of the geometry - for example the axis on the right side. On the other hand, some lines seem to be missing as on two of the screws. To clarify, this is what's wrong with it:
The freestyle settings are pretty common ones - you can't even adjust anything regarding the visibility.
I need to get rid of these artifacts. My guess is that I am missing precision somewhere - I tried adjusting the camera near and far limits, for example. I also searched for a way to set the depth resolution. If there is such a thing, I didn't find it. The result is the same for Eevee and Cycles, by the way. It also does not seem possible to composite the image by myself, as the new Freestyle Render Pass (new in 2.83.1) does not have a Z Buffer I could use to merge it to the rest of the image.