Couchbase Mobile using Sync Gateway is incredibly similar to Amazon Cognito. Even the native iOS and Android libraries have a similar feel to them (object-mapping with native objective-c objects, single call sync/replication function). Sync gateway has built in support for identity providers such as Facebook, but doesn't provide Google or Amazon identity support (this could easily be added with custom server code).
The big difference I am seeing here is Amazon Cognito doesn't yet have a story around syncing documents between multiple users/roles. Sync gateway has the notion of a sync function that providers developers with a hook to customize server-side syncing in a deeper way.
I'll be interested to see how Amazon developers the multi-user/role sync capabilities and integrates it with their native identity management solution. Could be very compelling down the road..