Earlier quoted context omitted.
I tried used Owncloud... and stopped. I tried using radicale... and stopped. The software was fragile, slow, and broke often. Maybe it's better now (a few years later). But at the time it was negatively impacting productivity. I've moved to Sogo. Zero problems after install. File sync is Resilio. Bittorrent sync was unusable, the new re-branded (and fixed) Resilio works fine. Just my $0.02
As with many things open source a couple of years is a long, long time; Nextcloud has improved a lot as far as I can tell, for me it's decently usable and surprisingly problem free. How's Sogo with regards to maintainability and upgradability?
It's really, really, nice.