[Quoth]

It does on the Kindle Touch and Kindle DXG. Empties the collections . Also if you have App, it makes duplicate connections.
It seems to only work properly with books you buy from Amazon. I did for tests put some stuff on Amazon's Cloud and have it delivered to my Kindle Touch and Kindle DXG. Unless it's actually already Mobi or downloaded from Amazon for Kindle, Amazon is worse at Conversions than LibreOffice + Calibre. There are other issues with Amazon Cloud too.

It's no substitute for your own storage, ask people that were using it for MP3s.

Values of eReader in approximate order:
1) read Classics from Gutenberg.
2) Saves massively on paper proofing own writing
3) Save Web sites and make them into eBook
4) Buy independent stuff off Smashwords
5) Other Indie publishers that supply free eBook with Paperback
6) Independent stuff off Amazon
7) Main publisher stuff off Amazon.

Least value is books from big publishers, rarely significant discount for eBook, so most of the main publisher books I get are still paper. My copy of Fiest's Magician was getting very tatty, so I did get that on eBook when it was on special offer.

Sync is very much an overrated thing, and in 30+ years of IT, I've never seen it be reliable. It's fast enough find where you were on another gadget. It's not like trying to match up 100s of emails.
Also Sync ONLY works for each vendor. No use between Aldiko or Bluefire (both preferable to Kindle App on Android), Kobo, Kindle, Calibre etc.
Sync only works with a Data connection on.

So my conclusion is that Sync is worthless.

Also Everyone I know apart from me only reads eBooks on ONE device, EITHER their phone app, their Kindle eInk or their Kobo eInk.