OS X Server: Multiple Mobile Homes syncing to a single Network Home Directory?
This is still a supported function.

It's something that needs care and tending (like any sync solution) when a burp causes problems, but Mountain Lion server hasn't made it better or worse than recent server versions (Lion, Snow Leopard) in the management and deployment of network home folders that sync.

If you allow multiple log in or true mobile syncing where a copy of the home folder gets literally copied to each Mac - then you will get a sync conflict dialog should there be three edits to the same file and the system can not systematically resolve the conflict.

In practice, the pre-deployed filters work well so that users become accustomed to not opening the same app on multiple computers once they start seeing merge dialogs. You can also modify the sync rules if you have particular problems with a specific software package/file or user.