If all you want is file syncing then Syncthing is really fantastic. I have a NextCloud installation on a Raspberry Pi 3, and getting it to work well took some suffering. If you use a MySQL database and php 7 it can actually perform pretty well though, at least for a small number of clients. A big issue with the Pi is that it shares the Ethernet and USB on the same bus, so there's a hard limit to your file access speeds. But for syncing between my own machines plus the occasional file share I am extremely happy with the setup.