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.
Can also check out Nextcloud. Yes, its more overkill than just calendar and contact sync but its not that hard to set up and self-host. It has an interface for you to fix your calendar and contacts graphically and works well with DAVx5, if on Android. Believe you can just add the CalDav and CardDav for iOS, but have not tried it personally.