It will be more customizable than Gnome with default settings. Tweaks won't be needed for certain customizations. It more likely will become an ecosystem like Gnome or KDE but that will take some time.
Not sure about the upgrade path from their current Gnome setup but I'm sure they will figure something out by the time stable is released.
Can't really comment on the older versions since I recently deployed the server. However from the past couple of updates, the background syncing has improved on the Android version. Before I had to open the app for it to sync but recent updates now upload media without me having to open the app.
For the app itself, works pretty well. There still seems to be bugs or weird UI changes like adding to albums disappeared when you click a photo or video but its there on the grid view if you hold the file.
Overall it seems pretty stable and it has a dedicated app in both iOS and Android. Use to use nextcloud with memories before.