Yup, checkpoint problem is pretty annoying. I beat the game on legendary a couple of weeks ago. I recently realized I never got the achievement for completing legendary. I loaded up my saved campaign and it gave me the checkpoint sync error. I figured I would have to restart the last level.... but I was wrong. I had to redo the last THREE levels on legendary.
Still no achievement.
Check campaign again, and get the checkpoint sync error. This time, it wants me to do the last TWO levels over again.

Thank god for the banshee trick when fighting tartarus; took me a LONG time to beat him on legendary the first time around (when I didn't know about trick).