Spoiler: show
Edited (off Brink's and KeyBlue's ideas)
ACU falls and loses 75% HP if it is not with a shield. If it is with a shield, it loses the shield and 20% of health. Fall time is 2 seconds. On impact ACU bends legs, squatting, and then returns to normal. That's hard landing. If it occurs over a mountains where a unit cannot land - ACU dies, game over.
If it lands on water, then it loses 50% hp if without a shield. If with a shield, it loses 75% of the shield. When it falls on water, I suggest using the animation it currently has for when it is hanging in a transport because it is doing a pencil dive. On seabed it stands up normally. This is soft landing.
If you do not like of the suggestion, make units drop faster from transports.