I use both, and find Syncthing a lot more useful in general.

Syncthing automatically my password databases (KeePassXC) between devices, ensures I make multiple copies of important files, allows me to move data generated in computers in the labs while I measure, and use it easily move files between my phone and desktop. There is a Raspberry PI in the network to keep everything in sync, but if it burns my files are still stored in other places.

Next Cloud I use to write down quick notes, organize spreadsheets, slideshow presentations, keep a calendar, and share files with others. So it serves a different purpose, which is to replace Google Docs in my case. And if my RaspberryPI shuts off I can't reach it.