The vast majority of upgrades have gone smoothly, IMO at least, see https://mon.syncthing.net/ for where I get my data.
If you have any super huge folders, check out this thread: Is Syncthing v2 with very large folders possible ? - #191 by gyver .
Otherwise, I think my experiences upgrading have been pretty representative: v2.0.0-rc.20 Upgrade Success .