"Sync on the other hand depends on three entities working well together, your computer, your internet provider/connction and Ancestry"
and the website that intercepts all messages between your computer and the Ancestry website : Incapsula Inc.
(as a side note, FTM2017 connects to the internet only when it needs to search Ancestry (via Incapsula) then it disconnects. This may explain some issues, especially during Sync.).