Migration assistant problems

Have you tried ethernet?

Move your content to a new Mac - Apple Support

Apple cover ethernet migrations on this page…

How to move data to your new Mac using Mountain Lion and earlier - Apple Support

You can avoid using a router if you have only one ethernet cable.

Otherwise I'd consider using target disk mode on the old device & see if Migration Assistant will use that for the source disk (still connecting via Thunderbolt).

Share files between two computers with target disk mode - Apple Support

I'm sorry I do no know what is failing. All I know is that multiple Thunderbolt attempts have failed, so I would try to transfer via another method. What are you hoping to transfer? Look on the disk to see if that has been copied, sometimes MA can 'fail' but has apparently copied the data. It is difficult to tell what may not have transferred in those cases.

Target disk mode will at least take the old OS out of the equation as the firmware is responsible for target disk mode.

Others have reported errors with MA/ Time Machine on Sierra with the latest Macbook Pro. Perhaps Apple have dropped the ball? There are reports of a future software upgrade but it is not clear what is actually happening or if it is just conjecture.

If MA fails once more I'd transfer the user data manually & then install the apps by hand. Ask if you need help with that, you can basically copy the user data into the /Users folder & then create a user account with the correct name - the OS should then setup the home folder permissions correctly, you just need to be careful with the account name setup.

P.S. I'd also contemplate creating a bootable USB stick so that you can erase & start again - it may be required if MA has migrated a fraction of the data.

Create a bootable installer for macOS - Apple Support

Or you could take both Macs to an Apple store & ask for help if one is local.