I don’t really like the way Nextcloud’s contacts and calendar sync works. That is to say, it’s unencrypted, it uses your system apps (both a positive and negative), and (on Android) it requires a third-party app to actually sync.
Using Proton for calendar is better, since it uses their own app, which is modern and up-to-date (not necessarily the case for Android system apps, or third-party calendar apps).
And using contacts for the same use makes sense in my opinion.
I wish Proton supported syncing the contacts to Android, and supported the calendar and contacts in the desktop bridge. (See this thread.) And I wish Nextcloud on Android didn’t require another app for their calendar and contacts sync.
So far though, Proton contacts is absolutely not a great option for me, but it’s the best option for me. And their calendar service doesn’t really have any issues for me.
Additionally, Proton is off-premise, so it’s safe from anything that may or may not happen to my Nextcloud server. Nextcloud contacts and calendars are not stored in files, so they’re not “backed up” to my Proton Drive as the rest is.
I could use the Nextcloud Android app to take backups of my contacts and calendar, then back those up, but it is not made for this.