Damn, that's right. Don't know what I was thinking. The problem here is that if the Overlord has to drop to sea-level at the start of its next move it'll automatically become entangled with the enemy ship with no means of easy escape (it'll have to try to disengage as per close combat rules). I don't know how that'll play in practice? You've had experience with this, right? How did it go?
An idea: allow for the Overlord at altitude to fly over enemy vessels. Then, after the move is complete, you would roll for the bombs dropped during the move. Only 1 enemy vessel can be attacked per move.
That would make it more tactically discinct, but not over powered I think. How about it?
I'm also a little concerned about the limited range of the bombing run. As soon as the Overlords speed drops to 6", paying 3" for ascending only leaves 3 for the move itself, which would make it hard to come into contact I think. But I may lack experience here as well. Just something to consider.
Oh, must have read the card wrong, sorry. Makes sense. As for vulnerability. Well it is only hit on a 6 so that seems fair. How about making the damage on the baloon never cause criticals (a la sails), but the 6th damage immobilizes the vessel until repaired?