I have 67,000 people in my file. I see people posting problems with sync on this board almost daily. Place names being replaced with completely different place names, etc, etc.
I am not prepared to subject my years worth of work to the disasters I have seen people posting here have had.
Although there are people who have had no problems with it, there are enough that have had problems to warn to take caution. Just look down the list of messages at headings to see the various messages of people with problems with it.
My suggestion is to not do it. If you have to have an on-line presence, upload it the "old" way (the share button in upper right of screen, check box NOT to sync), or upload a gedcom to WorldConnect.