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

Here's a way to get past it:

Michael, thank you! I understand, Idea with intersect is GREAT! Now everything is clear for me. Also Idea to select all edges in this case using naked esge script also is great!
BurrMan, good soltion too! Thanx)