I use mobius sync and I'd say the app itself is fine, you just have to open it whenever you want things to sync. That's one of the things I miss from Android. Also you can't sync your camera folder
Mobius Sync works really well, the only caveat is that it's not completely free (you're limited in the sync size unless you pay $5, but that's a one-time thing), and that while it can background sync, it's not continuous, and you'll want to open the app if you need to make sure something's synced.
Nope. I have a cloud Syncthing box that is accessible over SSH, and I use ShellFish to read/write my synced folders. It works okay, especially for lazily sending stuff from my phone to my laptop.