I haven't noticed synching probs on the Kindle app as long as I have devices on wifi or 3G/4G when stopping and restarting a book, with the exception of some personal docs. On the Kindle app on iPad, there's also a refresh button (a curly arrow on the upper right), that you can hit to make sure that a book is synched across all devices to the furthest position read.
Right now, I use the Kindle app on iPad 2 and 4, iPod touch 4 and 5, iPhone 4S, Samsung Galaxy Note I, two Kindle HD 7s, and rarely on Samsung Galaxy Tab 7 and a very old Kindle e-ink reader.
I also use Audible audiobooks on these devices, and haven't noticed synching probs either. I mention that because Amazon also owns Audible.