I don't think recumbents (at least high-racers) are necessarily unstable at slow speeds. My understanding was that the fundamental problem with recumbents in racing is that, going uphill, aerodynamics matters less. What really matters is applying torque efficiently. And humans are designed to apply torque vertically: we're good at it. So while on the straightaways and downhill recumbents cannot be beaten, they are poor at climbing.

What's frustrating is that we'll never get to see serious comparisons of the two, or of interesting combinations of them, in real races because UCI banned recumbents for the worst of reasons a hundred years ago.