Syncthing is a flawless Dropbox replacement in my experience. There's a big caveat that you need something like a NAS or your home server always online with it though as it's peer to peer sync only.
Syncthing doesn't support my older Mac devices anymore. I can't get Syncthing to work on Mac OSX Yosemite (10.10.5). And no, the device is not upgradable for a variety of reasons not worth going into.
Nobody talks about conflict resolution that Dropbox does..sync thing has an issue with conflicts because of its distributed topology. Editing the same text file on multiple machines inevitably resulted in a conflict that I had to manually resolved. I’m back with Dropbox for now.
This is something that nextcloud does really well. There are lots of sync tools on the level of syncthing, but my experience with nextcloud is a polished experience truly competitve with Dropbox on the complicated stuff like conflict handling, private/restricted shares, etc.