I'm aware of how a bielliptic transfer orbit works. See the long gap between burns 1 and 2 on the Wikipedia diagram? That interval is necessarily going to be more than half a year long, and potentially much, much longer. Spacecraft are pretty reliable once they're in space, but that's still a lot of time for something to malfunction. And if burn 2 doesn't happen, now you have tons of nuclear waste on an elliptical orbit that re-visits Earth's path every so often. At any given perihelion, the earth has only a small chance of being in the right part of its orbit, but eventually...

I'm not saying it's a big risk; the chance of a catastrophic launch malfunction is probably a lot greater. But it's definitely a long-term potential hazard that needs to be accounted for.