I have found that the playback position on content from Audible (about 9/10ths of my library) has behaved the same as content from other sources -- remember to ensure that the "remember playback position" option is checked doe your audiobooks in iTunes (otherwise it won't). As noted elsewhere, the way to share this position is by having the title on each device and by regularly syncing the devices with iTunes. (I don't play from the Cloud so I cannot comment how that works).
HOWEVER - as noted elsewhere in this forum - there are two problems that have occurred since Apple made changes to how audiobooks are played on Iphones/Ipads -- which is a) I have found that the audiobook is marked as Played/Read immediately it is opened in iBooks (so as before not when it is actually completely played) and b) since Itunes 12.4 was released I can no longer changed the played status of audiobooks in iTunes.
"Dear Apple please move the playing of Audiobooks back to the Music App, where all worked well, and please give us back the basic functionality that you allow to all other types of media which is to manage the played status of titles in our libraries in iTunes"
- Re: Why can't I change the played status of audiobooks in Itunes 12.4