I've definitely seen `sync` itself waiting/blocking (especially if you use fuse for something network based and disconnect the cable first), but whether it's guaranteed or not... that's an interesting question.

Edit: after some googling:

       On Linux, sync is guaranteed only to schedule the dirty blocks for
       writing; it can actually take a short time before all the blocks are
       finally written.  The reboot(8) and halt(8) commands take this into
       account by sleeping for a few seconds after calling sync(2).

       This page describes sync as found in the fileutils-4.0 package; other
       versions may differ slightly.

So it doesn't look like the writes are guaranteed to take place. Just a best effort + wait + pray :)