New Mac Pro: Setup/Migration Assistant never finishes
Also not really an answer but more a practical approach.

Another idea that requires manual work. I did this with a Macbook Pro and a MacPro 2006:


Boot your old MacPro into Firewire Target Mode (yes this works with "T" on power on).
Connect it to you new MacPro with a FireWire cable.
Mount all harddiscs
Copy over all data you need, but leave out the Library folder in your old homedirectory.
Do a fresh install of your favourite software, that is quicker and safer that copying them from the old MacPro or a TimeMachine Backup. You will definitely miss important files. (just my experience with MS Office)


Positive effect of this: you trashed all old things you really do not need. :-)

I even go a step further - On my old MacPro I set all files and directories I want to copy on a red label in the finder. When mounting the harddiscs, I immediately see what is important and needs to be copied. :-) And as a reminder of which software I will need on the new machine, I set the finder label "green" to all applications I will need on my new machine.