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.