This is frustrating, but it doesn't bother me too much on the iPhone -- I can just not use iCloud and periodically back up my phone locally. I don't really need to sync anything but photos because I don't create or edit documents on my phone.
On the other hand, the iPad is a very capable device for content creation, but I couldn't find a reliable way to sync files that doesn't leave them readable to Apple or another party. That'd probably be OK if I could use an iPad as my only computer, but I can't (or won't) today.
I don't really need to sync anything but photos because I don't create or edit documents on my phone.
This is one of the key points. You can plug an iPhone into almost any modern PC, Apple or otherwise, and download the photos and videos. But you can't do the same for other types of data without using one or more of an Apple computer, specialist software or iCloud. The middle option, which is the only one not entirely controlled by Apple itself, has a long history of attempts that proved not to work entirely reliably or last for long. And as you say, for an iPad, where perhaps more users want to do substantial creative work, the same issues apply.
I understand that Apple's current business model is heavily tied to locking customers into its ecosystem, but that does become a significant problem for people who just want a nice phone but also respect for their privacy, and I do think Apple should be called out for this just as I think it's fair to praise them for being much better on broader privacy issues than most of their competitors.
Almost anything else. iOS devices use the same protocols as things like DSLR cameras to allow plug-and-play access to download photos and videos (unless, ironically, you have iTunes software installed and it gets in the way). However, there is no equivalent to let you browse and locally download messages, notes, creative work, or any other type of data you might have on your phone with using one of the channels I mentioned before.
> a reliable way to sync files that doesn't leave them readable to Apple or another party
It's worth noting that nothing in iOS or iCloud prevents apps from encrypting data using a separate user key and then storing in iCloud. (Essentially, using iCloud only as a reliable substrate for syncing files it cannot read.)
That popular apps do not do this is likely due to lack of demand as much as anything. I am sure there are some content creation apps that do provide this functionality, if one is willing to search hard enough.
On the other hand, the iPad is a very capable device for content creation, but I couldn't find a reliable way to sync files that doesn't leave them readable to Apple or another party. That'd probably be OK if I could use an iPad as my only computer, but I can't (or won't) today.