No BO traits ever do anything unless they are assigned to stations.

Any station? Or a station that corresponds to the respective trait? In other words: do they really stack / does it matter where they sit? There seems to be some conflicting information here, hence I rather ask.

I select my BOs mainly due to style, but it would be interesting to know.

Any station. A human adding a repair buff doesn't need to be in an engineering station. He can be in any of the 3 classes. It's the same for a Saurian and efficient.

As far as my own experience, Leadership (space trait) only appears work for Human captains.

Human BOffs are listed with Leadership (as a ground trait) but it doesn't seem to affect anything, neither in space or on ground. So, in reality the question is, Why bother having any Humans in your crew?

We need more space traits. As it is the best crew would always be a bunch of Aliens (can have efficeient too), Saurians and a Borg BOff.