It takes a brave man to admit when he's wrong...and I was wrong. The cancel button WILL cancel the migration. What that means is that no more information will be brought into the system, but if things like emails, contacts, etc. are already queued up, then those will complete. That's why, Damir, when you hit the Cancel button there's still a progress bar: it has to finish what it's already started.
As for migrations hanging, we're looking into that. Going to leave this as "Submitted" for now, though, as we'll want to recreate it so we can find out what's going on.