2 statements are simply wrong:
1- when you defend from a stack, you can use less units than when you attack, since you need one unit per attack/kill (unless blitz) while in defense the same unit can kill more than 1 enemy unit.
When a stack defends, only the units in that stack counts. When you attack a stack in your territory, with only basic technology, all units within four tiles can attack. Mounted units can attack form even further. With Engineering all unit within six tiles can attack. If you want to defend by defending, you'll need large stacks all over.
2- when you attack you run greater risks than when you defend unless you are using an assassin or another clearly offensive unit, and even in these cases, it depends. Surely, the absolute statement that attacking is less risky is wrong.
When attacking you chose the attacker, the defender never gets to chose. So, if you are not prepared to lose a unit 20% of the time you won't attack at 80%. There is no similar option for defenders.
I more often lose high level units when defending than attacking, because the option of sacrificing a low level unit just doesn't exist.
The higher risk when defending isn't about losing a unit, it is about losing a valuable unit.
But the most important question is why am I forced on the offensive stance and can't choose the defensive one. Civ is a strategic game and you should be able to choose between different strategies and not be forced into one.
It's sad but, in my opinion, defence is woefully underpowered in Fall from Heaven II. If every unit in the game got +100% defence, it would
still be underpowered.
This isn't unique to Fall from Heaven. Vanilla Civilization suffers from the same imbalance, though not quite to that extreme. In fact, it is pretty much universal in all games.
As it applies to invisible units though, unless you accidentally conquered Nox Noctis from the enemy, you made the conscious choice of building it with a Great Prophet. Complaining that you are forced into an offensive stance after that is sort of like complaining that you can't run a solid specialist economy after building only cottages.