CalDAV-advertised subscribed WebCal is not imported
Nextcloud supports CalDAV, but it also allows you to subscribe to external WebCal-feeds (please refer to this DAVx5 picture for a graphical example).
When added in e.g. Apple Calendar, these feeds are automatically imported. DAVdroid / DAVx5 on Android also supports this:
Evolution however, does not import the feeds. The feeds have to be added manually. This is bothersome. It makes it hard to set the exact same colours across devices and every time you import a new calendar, you have to do so manually on Evolution on every device. Furthermore, it does not seem to be documented that Evolution discards the webcal feeds.
Gnome Calendar behaves the same in that it also does not import the feeds, I also created an issue over there.