As someone who has dealt with integrating and synchronizing data and accounts nearly my entire career, I can tell you this whole area is still in "there be dragons here" territory.

Here's a recent example of stuff Google did - merged my G+ circles into my contacts - ok, I can see how that'd be useful... but old or invalid email address for my mom and wife were substituted as first hits on the search when I entered the contacts in Gmail. Consequently, I've sent quite a few emails in the past few days where I'm getting bounces.

That's assuming you don't have to deal with improperly overwritten data, or time-sensitive data (ie, temporary details). Any degree of asynchronicity can create intolerable delays for example.

Abstractions can only get so far, especially for distributed synchronization across potentially adversarial vendors/systems.