Ok, so I'm using the Android app, and actually having less issues with it than most people.
However, after getting stuck at XX% and nothing getting it to finish syncing (restart Syncthing, reboot phone, etc), I took a closer look and realized what the problem is: apparently, it can't sync large files.
The file in question is a 3.53gb DVD ISO. Deleting it resolved the issue. Now, I'm not sure exactly what the problem is, but I can confirm that it's not a filesystem issue, as I've transferred the same exact file via both Resilio and FTP. It's really not a problem for me -- I can just stick it in another folder, as it's not something that's going to be deleted, but I figured that I'd point it out for the devs.
The suggested update for the FAQ is that Syncthing can, in fact, write to an external SD -- contrary to what is stated in the FAQ (https://github.com/syncthing/syncthing-android/wiki/Frequently-Asked-Questions). However, due to Android permissions, it has to be in the Syncthing sandboxed folder (in my case, microSD\Android\data\com.nutomic.syncthingandroid). But it does work, so perhaps a more specific warning in the FAQ?
That said, apart from the occasional crash and/or freeze and/or error, it works well for me. I'll really only need it when I add new files, and don't mind manually turning it on when I do so, so it's good enough for my use.
Some stats for you:
16655 items, 18.3gb, 77.5mb RAM
unfortunately, I can't find any reliable metric for CPU usage, but it hasn't seemed to kill the battery or slow the phone down.