Almost all safeties I have seen block trigger movement, not the sears. A cocked gun on "safe" is still a potential danger and can go off, ie after a knock or fall, despite being on "safe".
As for auto safeties, I can see their usefulness in shooting with a pair of guns and a loader. They protect from a fumbled gun change, and that is about it.
It is interesting to note that one of the most conscientious hunters ever, Aldo Leopold, ordered his SXS with no safety.
So auto or manual, it is about the same usefulness, which is close to nil.
