imapsync is slower than mbsync[1] (both in startup and per message) and doesn't support bidirectional sync[2]. It however handles message identity better than mbsync and supports using different authorization and authentication. For administering our mailserver, I first used imapsync and then patched mbsync to support the Auth header. I still use imapsync to sync the flags afterwards. I think both have high-level race conditions, so maybe take the server offline during a sync.

    [1] https://isync.sourceforge.net/
    [2] https://imapsync.lamiral.info/FAQ.d/FAQ.Two_Ways_Sync.txt