I still use Nextcloud for syncing documents and other basic stuff that is relatively simple. But I started getting glacial sync times consuming large amounts of CPU and running into lots of conflicts as more and more got added. For higher performance, more demanding sync tasks involving huge numbers of files, large file sizes, and rapid changes, I've started using Syncthing and am much, much happier with it. Nextcloud sync seems to be sort of a jack of all trades, master of none, kind of thing. Whereas Syncthing is a one trick pony that does that trick very, very well.