[tomsem]

Kindle apps sync fine for the most part. I sync between iOS and Android and my Kindles all the time. Normally it will sync even if you switch away from the app. But if there happens to be no wireless connection when you do that, it cannot update the location, and it cannot tell you that it didn't work (iOS requires app to shut down silently within a limited period of time, Android is similar). If you want more reliability and feedback, return to Home and manually sync.

There are other apps that sync between iOS and Android, let you upload your own content, and sync. Google Play books and txtr come to mind. But they are likely no more 'reliable'.