Anecdotally, I've been managing a Syncthing network with a file count in the ~200k range, everything synced bidirectionally across a few dozen (Windows) computers, for 9 years now; I've never seen data loss where Syncthing was at fault.

Good to know. I wonder what the difference is. We were doing things like running go build inside the source directory. Maybe it can't handle write races well on Linux/MacOS?