My experience is that dogs from working lines are much more stable, than (purebred) dogs out of cute pets or sweet dogs of unknown origin.
The ancestors in working lines have proved they can handle stress/threats to a certain degree!
Dogs from working lines are supposed to have lot of drives and they can be a handful, if their only choice is, to get the outlet of the drives in the living room and not on the training field.
Really, it’s nothing… to be compared to all the work one must do, just to cover e.g. the symptoms of a nervy, shy or fearful dog.
The ancestors in working lines are good proves of working abilities, sound nerves, temperament, tractability and that they can to act rational to everyday situations (they have seen worse threats than kids
)