Apple's online services take a lot of heat for being unreliable. The details have shifted over the years, but the basic form remains the same: Changes made on one device sometimes don't show up on others. The situation improved somewhat since Apple introduced CloudKit and started migrating apps to it, but problems still occur. It happened to me just recently: I added a note to the Apple Notes app on my iPhone, then switched to my Mac, where Notes was already running. No note. Fortunately, I found a quick fix: I quit the Notes app on my Mac, re-opened it, and bingo – my note appeared.
Quick fix or not, Notes felt broken. When it comes to this type of situation, Apple's vaunted "it just works" philosophy just...doesn't work. I assume that re-launching Notes prompted the app to sync with iCloud, which when pulled down my note. But many users might not know to try re-launching the app. All they'd see is an app that hasn't properly synced their changes from one device to another. That's a pretty quick way to reinforce the "Apple services aren't reliable" meme.
Apple makes things harder for themselves by omitting any kind of sync status indicator or manual sync button. Consider the Kindle app as a contrast. When I open a book on my iPad, it almost always offers to sync my reading position to the last page I read, even if that was on another device. Every now and then, however, it fails. In that case, all I have to do is open the menu and tap the Sync button to trigger a manual sync. Problem solved. As a user, that feels much less "broken" than the Notes app, where there's no obvious solution to the problem.
I get where Apple's coming from: They want sync to feel seamless and effortless, something that happens without the user having to think about it. But the fact of the matter is, it doesn't work that way. Sync rarely does, because it's a very hard problem to solve. Apple isn't alone in having trouble with it by any measure. But by not giving users a way out when problems do happen, they increase the perception that Apple services don't work well.
Apple probably doesn't want users to start thinking that they have to tap the Sync button in order for their data to be synced. Indeed, that seems like something some users might start doing. For example, lots of users force-quit apps from the app switcher when it isn't necessary. But Apple lives with it because it offers a way out of a rare and undesirable situation. That's far superior to leaving them stuck wondering why the software doesn't work as intended. (In fact, you could argue that unnecessarily force-quitting apps is more harmful than unnecessary syncs, since those apps won't be able to continue running background operations.)
Adding an unobtrusive manual sync button to cloud-based apps could go a long way to improving the perception of reliability. Obviously, the ideal is for sync to work automatically all of the time. But it'd be better for Apple to bow to reality that sync doesn't always work perfectly than to continue letting users get stuck.