My nextcloud instance killed itself during an update. I wrote a blog post about it and people regularly write to thank me, so it wasn't an isolated instance. Syncthing is much simpler, and practically bulletproof. It really just works.
Despite that, nextcloud/owncloud do what I want/need and I'm not sure if Syncthing does. So I keep stubbornly re-trying and just spin up a new instance when the old one dies.
I just spun up a new instance, I'll see if keeping it simple works for me. It's also on a VM now so I can take a snapshot and roll back to that if it breaks rather than relying on the faulty backup steps.