That’s not good that Master broke for Debain & Ubuntu… Maybe this means it’s time to move away from Debian / Ubuntu based distros if it’s going to be difficult to get darktable installed?
The master branch comes with no guarantee of stability and might corrupt your database and XMP files, result in loss of data and edit history or temporarily break compatibility with previous versions and commits.
How dangerous is it? Most of the time, it is fairly stable. As with any rolling-release kind of deployment, bugs appear more often but are fixed faster too. Sometimes, though, these bugs can result in losses or inconsistencies in the editing history of your pictures. This is fine if you don’t need to open your edits again in the future, but maybe not if you manage an estate.
I have been using (OBS build) master branch of darktable for almost two years now for my daily work and have very rarely experienced problems both from the master branch itself and from OBS as the builder. And when there were problems, they were not so severe that they prevented the daily use of darktable.
I must say that for me it is amazing how much commitment is invested in stability and smooth functioning already in the developer version.