A few days ago, I ran into an issue tracing a line onto a 3D surface to trace with an endmill. My sketch geometry would only project a portion of the line segment onto the face and not the entire thing. I also attempted to use the project toolpath directly, but that too didn't work that well due to floating off the part's surface and not cutting it as the 3D face would get steeper.
I sent out an email to Autodesk about this problem and sent in a simulated file of the two problems I was running into. They got back very quickly and even provided a walk-through video on how and why these problems occurred.
The gist of the sketch project-to-surface problem was that by default, it would project the line to the closest points on the surface, so if I had a line at an angle, it would only take a tiny segment. The solution is to change the project method from the closest-points to an along-vector, which has you select an orientation on which to project the entire line segment direct onto your surface from the plane you select.
For the project-toolpath, the problem was around the tool's engagement with the material. Since I was using a ball endmill, the tool's centerline would float over a steeply curved surface, as the tool would be engaging with the side and not from the center point alone. The problem isn't really a problem, it was just the way my workpiece was setup, and if the toolpath engaged as I had in mind, it would gouge the material pretty severely.