One subtle point to note is that OW fire on ground formations may occur before or after the troops disembark, while OW fire on air transport and planetfalling troops only occurs after disembarkation.

1.7.5 Transport Vehicles.

Transported units may disembark at the end of any move after the move in which it was picked up. This happens after any overwatch shots but before the moving formation shoots or assaults.

Where does it disallow the firing of Overwatch before the troops disembark? the only reference I found was in the official errata, where it says “Landing does not count as movement for the purposes of triggering enemy overwatch fire. Disembarking triggers overwatch fire as normal.â€Â

This is not the case.

A formation that is on overwatch may choose to shoot
immediately after an enemy formation completes a move or
unloads troops, and before the target either shoots or assaults.