Aliens not too far from the Skyranger will be facing you with plasma weapons aimed at the doors. Aliens further away will ignore the Skyranger and continue on with the mission.
I like the random TU idea to balance things out. I'm also as gung-ho about this as Blacklibrary is, so give us options, I want to experience both ways. As the Xenonauts creator said, Aliens are tougher and better than your operatives, but you outnumber the aliens in every mission, so you can lose a couple of men and women but still win by the numbers. So I also want D-Day scenarios in my X-COM with 100% TUs, once in awhile.