Anything "stateful" like a database breaks this paradigm.
I have nothing against databases used as cache that can be "re-filled" upon re-creation, but I believe anything holding business critical data shall be held outside of a kubernetes cluster. Why, because being one command away deleting your StatefulSet, Helm Release ... etc scares the shit out of me.
You can of course minimize the risk with correct RBAC, ensure proper backup/restore migrations but that require lots of staff and efforts I can't spare.
So until I can be reassured that I have all the tooling that can recover rapidly any catastrophic failure/mishap, and that all this tooling is tested monthly, I enforce using managed databases services.