Let's say I want to share a file with a friend internationally. First off, while there are some reasonable UXes for Syncthing, a lot of them are pretty basic, or rely on running a daemon and then connecting a web browser to Localhost to see what's up. Once they get it set up, then I have to actually set up the share with them. To get them hooked up to my share, I have to send them a 50+ character ID string somehow, which they then have to input into a UI that's far from easy to use. The key is much too long for me to want to read over the phone, and putting it in a chat somewhere means that if that chat ever leaks, my private key for my shared folder is out there. They offer a way to send a QR code, but that has the same leak risk, and scanning a QR code on the computer you're already on is awkward.
In short, it's a great tool, it works well in general, but the initial setup is pretty cumbersome if all I want to do is send a couple files to someone.
Additionally, I've had a couple time where even just syncing between my own devices broke. I think it was something where files were changed on both sides and the reconciliation algorithm got confused, but it was hard enough to debug for me, with direct access to both devices, and decades of experience running and programming computers, that I'd never want to try to debug that over the phone with a friend.
