I went through a horror scenario that scared me when I bought a new iPhone and my old pictures and videos for some reason did not sync over to the new device. I thought for a few brief moments that I lost 2 years worth of pictures and videos and that was enough to scar me permanently.
I now periodically back up all my pictures/videos from my iPhone and iPad to a folder in my Dropbox. Seeing all the files listed out in my folders gives me comfort, because I know there's no attempts at cleverness going haywire in the background, erasing and moving things around when I click "yes" or "no" to some popup question that I don't fully understand.
So I'm not sure what the right answer is. Sometimes I wish it wasn't as much magical as completely transparent about what's happening. Maybe there could be some nice UI that shows exactly what is stored, and people could somehow drag and drop data between devices?