FolderSync does the job, that’s right.
But Folder Sync does not read the catalogue-files from Owncloud and Nextcloud.
So, the FolderSync App always parses the whole directory-structure to find changes.
I am using Folder Sync Pro.
I have a structure like this (SD-Card NOT formated as internal storage):
documents
- Audio
- Pictures
- Documents
- …
Content: About 9.000 files with about 70GB data.
I upload a new picture to the server and at least 1 or 2 hours later (after 1/4 of my battery consumption is gone) Folder Sync fails due to the amount of data.
So, forget Folder Sync.
Till last week I used Owncloud since 6 years.
But I got annoyed about the missing feature of 2-way-folder-sync like on the desktop-app.
Then a colleague told me he uses nextcloud and they have the feature.
And indeed, there was written (Mobile Sync App):
Blockquote
The apps allow you to access, sync and upload your data and feature instant upload for fotos and videos, upload management and more features.
Blockquote
But you have to create a home-folder where you can download server-folders…no possibility of specific folder-pairs like on the desktop-clients. No difference to Owncloud.
I think there will never be such an important possibility like folder-pair-based 2-way-sync with the native nextcloud or owncloud-client.
I wonder what may be the problem to say: Hey, Nexcloud app, here is my local folder “storage/3452-335/dcim”, please sync it the the server-folder “smartphone1/dcim”.
Perhaps some developer can explain that, please?
By the way…
After one week of migrating from Owncloud to Nextcloud I came to the conclusion that is was not worth the effort 
Greetings
Patrick