For pure sync between machines, syncthing is amazing and pretty simple.
Nextcloud/Owncloud/Seafile/etc are much more involved, expansive pieces of software with many integrations and possibilities available, which is great if that’s something you want to be the sysadmin for but way overkill for just syncing files between a few of your own machines or between a small group of people. They’re more like a replacement for the google drive/docs suite and better suited for things like offices or if you want to get your whole (extended) family off google drive/dropbox because you can create multiple users and have different levels of admin privileges, shared calendars, document editors, etc.
Fwiw I used to run a fairly modest nextcloud instance but eventually retired it because it was overkill for my needs, required too much upkeep and I was always stressed that if I messed something up, the files in encrypted database would be inaccessible to me.
Now I have a simple little master syncthing server that my machines all sync to and copies of the files are always available on all machines.