That's no crash but an exception MM is not able to handle. So it aborts the task.
MM booleans allow (against other software) to get a boolean result also if the involved objects are not watertight. The only limitation is: The holes ( open boundaries indicated by those blue seams) must NOT be on the intersection line between both meshes. As long as you do not fix those holes booleans can't work.
Instead of BooleanUnion I recommend to Combine both (maybe more) surfaces to one object and run MakeSolid (with sufficient SolidAccuracy and MeshDensity) on that object to merge the surfaces.
