> With enough automation the crew [of a generation ship] could be quite small.

Other than the engineering challenges, we don't have the faintest idea how a really long-term crew is going to work socially and psychologically in a constrained, isolated, artificial environment.

The closest analog are the naval/polar expeditions, which were unisex and under military or hierarchical command, none of them longer than a couple of years. Some perished (Franklin), some pivoted (Shackleton w/Endurance), some failed with loss of life (Scott), some worked (Amundsen). But all had options unavailable to a ship in vacuum, and were on an incomparably shorter timescale.