Server also can be hit-and-miss if you've got any irregularities in your setup...I had a rough upgrade from 10.04 to 12.04 a few weeks ago (died halfway through, system left in half-upgraded broken-but-bootable state). In the end it turned out that it was due to a no-longer-maintained package (gitosis) which I'd replaced some time ago (with gitolite) but I hadn't actually removed the gitosis package and there was some hangup about deleting the gitosis user.

In fairness, after manually resolving the problem with removing gitosis and cleaning up a couple of other half-upgraded things, the upgrade process did recover and finish successfully, but it certainly wasn't painless.