Best example I can think of is when they shipped an update that they knew would cause corruption on ZFS disks and didn't warn people sufficiently to not update if they were using ZFS. (A warning was buried in the release notes, but such a serious issue should have held back release. Most normal people don't read fifty pages of docs before each update.)

I don't hate Ubuntu at all, but that incident showed a level of blithe carelessness to user data that is unacceptable and reduced my confidence in them.