My machine also failed to boot after the update. I could not determine exactly what was causing the issue, only that after upgrading and running the migration process the boot tree would not be brought up completely.
Debugging the live system was not possible because things like /dev/pts were not being mounted and sudo refused to work because of this (I have root logins disabled). I started by removing my custom services (which the new version was not happy with) but in the end I had to rebuild all the trees again from zero because there was some sort of inconsistency I could not fully understand.
I posted a new thread in this section a few minutes ago with the outline of what was needed to recover my machine, for those who want to try one last thing before going for a reinstall.