This one release per month is the normal release cycle (https://github.com/nextcloud/server/wiki/Maintenance-and-Release-Schedule). You have to look much closer to the changelog to see the fixes related to stability.
for some reason i was kinda forced to upgrade to nc 15 yesterday and i am really positivly surprised of 1 - how smooth the upgrade went and how fast nc 15 is running. so far without any detectable problems.
i’m running nc 15.0.4. on a rpi3 with php 7.2
i really love it.
so to put an end to the thread-heading question: Is Nextclound 15 “stable”? i’d answer: yes, of course.
and that would finish all of the above discussions. i think :o)
Not inherently less stable than previous versions. Let’s close the topic here. If there are other interesting topics to discuss, please open a new one.