Patching and Plastering - skim coat or start over?

I have removed a lowered ceiling in my kitchen (soffits over the old cabinets) but getting the whole ceiling to be smooth is turning out to be quite a chore. Is there some trick to doing this? Is it more than a novice drywaller can handle? The new wallboard on the ceiling is the same thickness as the old but the joints did not match up evenly. In some cases, it was off as much as 1/2 inch. I taped and mudded it but it is clear it will take lots of mud to make it so you can't see the transition. Any other ideas? Suggestions?

1/2" is a bit much. I would reset the wallboard with shims to bring the edges together.