The syncing code is rather cludgy when it comes to its scheduling. An exaggerated cases:

* Synching starts at 01-02-2012 12:30:00. $last_sync gets that time.
* Synching for list 1 starts, fetches subscribed and unsubscribed users since $last_sync. Takes 20 minutes.
* User Anna unsubscribes, it is now 12:50:00
* Synching for list 2 starts, fetches subscribed and unsubscribed users since $last_sync. Takes 40 minutes.
* Synching stops, saves "emf_last_sync_subscriptions" with value 01-02-2012 13:30:00 (one hour later).
* On next sync, Anna will not be fetched..

Especially on more busy platforms this will be a problem in many edge-cases.