Just installed Syncthing on my Scale server. It looks like it doesn’t have users but rather folder IDs that are then used to sync devices. One of the cool features of Nextcloud is the ability to share files with other users. Can this be done with Syncthing?
Oh forgot to add that the last case that you mentioned, where multiple users sharing a PC, and keeping the folder in sync with all, is not straightforward. This needs another always-on (server like) device.
At least in Windows each user gets a different Syncthing ID. So if you sync the file with an always-on device, the other user can get the update when they come online from that.