I once maintained an extension (grauphel, to sync notes between tomboy on PC, Android and N900) but stopped eventually because every new major Nextcloud version brought API breakages that required me to adjust my app.
This meant I had to do unnecessary work every six months just to keep the app installing on the latest release, although the app itself was feature-complete years ago.
The ChangeLog was pretty sad: For several years, it only contained entries āMake compatible with Nextcloud Xā.
In the end I would have had much less problems and unnecessary work if I had written a standalone web application instead of a Nextcloud app.