I think they’re both good for different use-cases. I use nextcloud myself on a truenas system. I sync things like my pictures to nextcloud, and delete them from my phone after I’ve sorted them into the correct folders.
This way my data isn’t clogging up my phone and other things, is still available from anywhere (as long as my home internet doesn’t go down), and it’s still safely stored on redundant storage.
This does take a bit more setting up than something like syncthing, though it wasn’t very difficult at all. Basically install the docker image, tell it where my data goes, and set up a new dns record if you want it publicly accessible. I personally run it through a zerotier network so I don’t have to do that.