I can't remember all of the different problems now, but one I do remember is that if you had a typical set-up with mirrored (RAID1) drives but the boot-related partitions cloned rather than mirrored, one of the bootloaders got upgraded but not the other. That is, the drives were left out of sync and booting from one of the drives wouldn't work properly if the other failed. The thing that really concerned us wasn't so much the specific details here but that this was essentially a silent failure in the upgrade process, combined with a potentially catastrophic failure in a basic system function as a result.

I think there is quite a big difference between the theory of updating a couple of sources files and running a couple of upgrade commands and the practice of manually checking things like basic RAID configuration and reinstalling missing bootloader updates. This time around, the fact that Jessie uses systemd made the discussion for whether to even try a dist-upgrade a very short one, because literally everyone in the room agreed that the probability of failures was too high for that strategy to be worth considering. The substantial discussions were more about migration to fresh machines relatively soon vs. sticking with 7 at least until we know the LTS situation.