Hello,
Keep in mind that you can host your files anywhere and even on proprietary
systems but files must be encrypted with your own key on your own computer
using only libre software then you can upload/sync on other devices.
Also avoid default encryption with nextcloud (more info about server side and
client side encryption nextcloud encryption
https://nextcloud.com/blog/encryption-in-nextcloud/)
I'm not saying that the way they use encryption is bad... but when you host a
nextcloud web site on other web hosting providers you don't have access to
the private and public key... it is secure when you use nextcloud on your
devices.
If you use your own server I think its ok because the keys are stored in your
machine. Maybe someone in this forum can help about encryption, i'm not an
expert :).
For the auto sync on the mobile I think it is much better to manually start
syncing. its saves you battery :p
