I've personally had no major issues with syncthing, it just works in the background, the largest folder I have synced is ~6TB and 200k files which is mirroring a backup I have on a large external.

One particular issue I've encountered is that syncthing 2.x does not work well for systems w/o an SSD due to the storage backend switching to sqlite which doesn't perform as well as leveldb on HDDs, the scans of the 6TB folder was taking an excessively long time to complete compared to 1.x using leveldb. I haven't encountered any issues with mixing the use of 1.x and 2.x in my setup. The only other issues I've encountered are usually related to filename incompatibilites between filesystems.

I will say I specifically don't sync git repos (they are just local and pushed to github, which I consider good enough for now), and I am aware that syncthing is one more of those tools that does not work well with git.

syncthing is not perfect, and can get into weird states if you add and remove devices from it for example, but for my case it is I think the best option.

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.