Booleans limitations

Limitation	Workaround (if available)
Booleans may fail if an intersection lands on overlapping components belonging to an input object.	Workaround #1: Select Mesh > Merge to merge the overlapping components. Workaround #2: If you don't know which components are causing booleans to fail, turn on Use Thresholds in the Channel Box to merge vertices on all input objects.
Booleans may fail if an intersection lands on components from different input objects that are overlapping.	Nudge one or more of the input objects (or nudge their components), so nothing overlaps at the intersection. Tip: Run Mesh > Merge on the result to avoid very tiny edges between the previously overlapping components.
Booleans may fail or produce unexpected results if an intersection lands on illegal geometry (non-manifold, lamina).	Delete or resolve the illegal geometry.
Performing a boolean operation with more than one input object can result in objects being left out of the calculation or the operation failing altogether.	After your boolean operation has failed, you can try opening the Channel Box and setting the Classification attribute for polyBoolean to Normal. Normal Classification treats meshes like open volumes:

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