Another way you can see if there are intersection problems is to select the objects you're trying to boolean and do a Construct > Curve > Isect , which generates all the intersection curves between 2 objects.

If you do that in this case here, you'll see that because the surfaces just very slightly wiggle through each other there are complex intersections in those shallowly intersecting areas:




It's then very difficult for proper final trim boundaries to be created from this type of messy intersection results.

- Michael