In the past, they gave away a few details:
There have been a few discussions recently about testing, however not much feedback from Nextcloud. And from previous bugs, we don’t know if there were analyzed and their internal testing was adopted to catch similar problems in the future.
In the first releases, my incentive for testing was to satisfy my curiosity about new features. It gives some time to check them out, find possible problems etc. before running in productive environments.