Allright I had two times where the script didnt work. (and two where I got shot down, but most of the scripts did work)
The first time, two never spoke after takeoff. He did answer the pre-takeoff briefing thing but after that just went silent. I had set radios correctly. (by mission 13 in one of your campaigns, you know those things by heart :D)
The second time, the script also didnt fire (as in: no underway talking), but once I flew over a waypoint one of the wingos anchored and overlord spoke for a bit. Sadly once I had annihilated everything in the target area except the L-shaped building the helo was going to land on, nothing happened. No helo ever showed up.
So. Broken sometimes, not always. What a B. to test. I suspect a trigger zone at the start of the mission is narrowish. IE you dont fly the flight plan exactly by the numbers, script breaks.