I’ve noticed that if your invasion fails while using a destructive bombardment, no improvements are destroyed but you still get the combat bonus during the combat. So the exploit is to invade a large planet with one transport at a time using bombardment and when the planet is about to fall, use enough troops to do a normal invasion.
The Paradox is the game was designed so that you could not bombard without invading. They could have the improvments destroyed no matter who wins but this would still not stop players from sending in suidie missions to bombard a planet. The only way to stop the exploit is to allow bombardments, which defeats the original design.
