Thanks for all the warnings about what can be lost. I recently did a restore of a backup tree on FTM after a failed sync, and ended up losing ALL my default photo settings on my Ancestry tree, much to my dismay. And, I finally had to re-download my tree again anyway after that.
Now, can't sync at all on my largest tree, so will have to re-download it again when I can schedule the time to do it. I've done that many, many times.
The sync process seems to work OK for a few people, but it is overall, in my humble opinion, a tremendous source of frustration for those of us who have spent many, many hours trying to get it to work.
I've been in touch with Ancestry support more times than I can count, always supplying screen prints and detailed step-by-step traces of what I have done (I used to test software professionally). I sure wish they would get these issues fixed so that it would work as it should.