M Tidal Locking
I agree that that is a serious problem. Some studies suggest that a decent atmosphere will not freeze out on the darkside and that a habitable ring will surround the too hot sub'solar' point.
One astronomer calculates that the # of habitable moons of giant planets surrounding M stars would greatly exceed the # of habitable planets in stellar habitable zones. As they orbit their primary giant Neptune or superjovian, these putative ~ Earth sized moons would have days ranging from tens to hundreds of hours. I don't buy his optimistic #s (mainly because other papers hypothethize that even for big fat planets Earth sized, .1 Earth mass and above, moons may be rare) but it's worth considering.