This is actually perfect timing.
I have been trying desperately to find a way to sync files between my various devices. For "work" (i.e. non-gaming) stuff, I use a combination of a laptop downstairs, a desktop upstairs, and my iPad. Schlepping stuff back and forth on a thumb drive was fast becoming obnoxious. But I don't really want to sell my soul to Apple (via iCloud) or Google Drive.
Attempt 1. After investigating various cloud hosting services, I decide to try Cryptomator. It's a Java-based program whose purpose is to encrypt files and then sync them to whatever service you're already using (e.g. Dropbox). Unfortunately, I tried it on 2 separate machines (both my Linux ones) and it simply doesn't work. "Locking" the vault doesn't actually encrypt anything, and I got conflicting messages on various fora about exactly which directory I was supposed to point the sync service at. I was already skeptical because their (paid) iPad app doesn't have proper Files integration, and the developer's responses to requests for this have basically been "it's too hard."
Attempt 2. Next, I decide to try Mega. They offer 50GB for free, and say they have sync apps for all platforms. But their Linux app sucks; if you're not using a .deb- or .rpm-based distro, you get a tar file that's apparently meant to be extracted to / (because it contains subfolders like "etc" and "usr"). Once that's sorted, it does actually work. On the other hand, their iOS app is hot garbage. It can sync photos and videos automatically, but nothing else. To upload a file, you have to:
1. Run the app.
2. Open a menu
3. Select "Import from..." (You'd think it would be "upload," but that only applies to photos or vidoes).
4. Select "Browse" (which is the only option at this point in the sub-menu)
5. Go find the file and upload it
Attempt 3. I finally said fuck it, and signed up for my own VPS with Vultr. 1GB of RAM, 25GB of HD space, plus daily back-ups is $6/month. They have a few app packages already, as well as a few OS installs. They have a NextCloud app ready to go, but it's based on Ubuntu 16.10, and I've had too many bad experiences with Ubuntu to go that route. So instead I install CentOS to get my own NextCloud setup.
This takes several hours, due to an unfamiliarity with CentOS's quirks, 18 different versions of PHP7 out there, various VPS-related issues (such as being unable to enable SELinux without bricking it or being unable to SSH in after changing the port), and other miscellaneous nonsense.
But I finally have a working, encrypted NextCloud setup. Web app works, and it's not the fastest thing in the world but it's fine. Install a couple apps (calendar, notes, and tasks), and off we go. Except the iOS app basically refuses to work. It can login fine, and it has proper Files integration. But it can't sync. Trying to do so just gives me the spinning circle until it times out. Web interface works fine from Safari, but this too defeats the purpose.
So despite hours, a little bit of money (Vulr charges by the hour) and a not insignificant amount of profanity, I am no closer to actually having a syncing solution.