Big advantages over own/nextcloud:
* Easy updates that work. When I tried owncloud, nearly every update I had to fiddle around with the database to make it work. Didn't have a single problem when updating Seafile so far.
* Block-level sync. Meaning if you change one line in a 10MB file, it will only update that part. Last time I checked, owncloud did not support this.
* Sync just works. When I tried owncloud, folders disappeared, folders were duplicated. Never had this problem with Seafile.
I've run ownCloud for some time now and I don't see the reliability issues that some people claim. I am running it on TS140 and not something small like a RPi, so maybe that makes the difference.
The first time it happened I shrugged it off as a fluke. The second time I became gunshy and would disable it any time I was working, then enable it after. Not a big inconvenience but a hassle and something I had to remember. After a while I stopped using it. YMMV, of course.