If I'm not mistaken, there seem to be better cross platform options in Parse and Dropbox. A case can be made for the "walled garden", iCloud doesn't seem to be one of them.
There is indeed an opportunity here for other cloud providers - but "better" is fuzzy. The nice thing about iCloud is that once you set it up on a device, there is no need to configure it or authenticate ever again.
It "just works" (if it actually worked).
With Dropbox, either you are storing everything for all of your users (and paying for it), or you integrate into the user's Dropbox account - which would necessitate a fancy ol' login/authentication system. Far from seamless, and relies on your users subscribing to something that is popular but far from ubiquitous.
So... either pay through the nose for all your users' storage, or force them to jump through some circus hoops on their own. Neither are great.
I'm not too familiar with Parse, but Dropbox is not solving the same problem as the feature of iCloud that developers are complaining about. Whole file syncing is comparatively easy from the server side. If there a conflict between the server-side file and the client-side file, the app has to work out how to handle it. Core Data (basically a local database) syncing tries to handle conflict resolution automatically, since it has knowledge of the data structures. That's tricky, and that's the part that doesn't work well.
I haven't seen many complaints about iCloud's file syncing features, which are what Dropbox is the direct competitor to.
While Dropbox is the most popular shared-drive mechanism out there, there are alternatives to it.
1) Google Drive - took my a long time before I even tried it, but it works about as flawless as dropbox for my needs. I actually like it a lot as I use Google Docs heavily, so it makes it easier to use those tools.
2) SkyDrive - offers the most base storage than the other. Has nice support for MS Office products.
3) AeroFS[1] - While it's still in beta, it looks promising (it has limited (android) to non-existant (iphone) support). [1] https://aerofs.com/
The great advantage to iCloud (whatever issues there are) is that it’s already there, integrated to the system. There’s almost nothing your users have to do in order to use it, quite unlike Parse and Dropbox. Granted, “almost everybody” has a Dropbox account and it’s free, but the nice thing about Apple devices is that they mostly work (or used to work?) even for people who know almost nothing about computing.
It "just works" (if it actually worked).
With Dropbox, either you are storing everything for all of your users (and paying for it), or you integrate into the user's Dropbox account - which would necessitate a fancy ol' login/authentication system. Far from seamless, and relies on your users subscribing to something that is popular but far from ubiquitous.
So... either pay through the nose for all your users' storage, or force them to jump through some circus hoops on their own. Neither are great.