Anachrony
Why does the rulebook advise against mixing Pioneers of New Earth with Doomsday?
It is a balance and satisfaction issue, not game length: Doomsday rewards competing for experiments, while Pioneers is a race with a semi-random payoff, so heavy investment in one leaves too few actions for the other. The designer says Doomsday generally mixes poorly with other modules and expansions, though he left combinations open.