So far, the only problem I have run into is the insistance that NetworkManager be installed.
Insistence as in removing it causes bad things to happen? Prior to 12.04 I always removed it for desktops because it was more trouble than it was worth; starting with 12.04 I've tended to leave it enabled.
I did manage to get the desktop installer to install on a software RAID. The trick was to boot the live CD into "try Ubuntu" mode and setup the mirror first, then click the install icon. Do a manual setup of the disks and it will let you pick the the md partitions. There is one catch though. The install will fail at the very end when it tries to install grub. Apparently the installer doesn't install the mdadm pacakge and grub requires mdadm to configure itself. Open a shell, chroot into the new install and install the mdadm package, then run the grub setup command (the exact command is in the live CD syslog). There is a patch to one of the grub scripts
to get rid of the "disk filter writes not supported" message you get on reboot.
Ahh, that's good to know.
Did any of these systems have a UEFI BIOS, by any chance? UEFI caused me a lot of grief with 12.04, it would be nice if they've cleaned that up.
I have one more machine to upgrade, but that one is going to be a bit more dicey. It is the house file server and is running 10.04. Yes, it has been running four years without being touched. I'm thinking it will be a two step -- upgrade to 12.04 first, then go to 14.04.
In your shoes I'd probably just do a fresh install to another partition and copy the server configuration over.
My home server is still on 9.04, so I probably don't have a choice. The hardware is old enough anyway that I'm planning to build a complete new one and swap the whole server out.