The main problem is resynchronizing after a period of staying offline. Full sync requires downloading full message list with their flags from all watched folders.
If you have two devices, and both have downloaded a message, then device 1 went offline and device 2 deleted one of the messages or marked it as read, device 1 will not receive any notification of this event. It will have to download all messages after reconnection or rely on QRESYNC extension which is usually not available.
See my reply here:
This is not impossible, but it’s completely different compared to how DeltaChat operates today.