Effects of multiple nuclear explosions (blast waves / radiation - aircraft are sensible things, even "hardened" warplanes), their home and secondary bases being primary targets for a soviet retaliation strike, soviet anti-aircraft activities, either ground-based or with fighters (B-52s aren't exactly stealthy), and the fact that even if they make it back somehow, everyone they know is dead or dying horrifically, and the likely chance of a decade-long nuclear winter with nothing growing...

War is hell.