And once you do a Construct > Curve > Isect, you can see the high complexity of what's trying to happen there just by how many separate little segments are generated - doing an Edit > Separate on the intersection results and you get 609 objects.

In order for the Trim to go well all of those 609 little segments have to make a well formed closed and non-self-intersecting loop that divides the surface area of the joined knurls into distinct regions. If there was any problem in even just one little area that would prevent the full Trim from working properly.

Since the intersection results get joined together into 18 pieces at first, there's probably around 18 different locations along the trimming boundary that have some of those problems (either grazing an edge making for an overlap being generated or a gap as shown in above screenshots) that will need some individual attention.

If you had a more regular 2D cutting curve instead of a 3D swooping one it would probably be less likely that you'd have some of these issues like as much grazing / edge-overlapping-cutting surface type stuff.

- Michael