Orcs – discussion about the race.

Hmm... That's quite a bold statement about the invincibility of the Orcs. I suggest you read the information in the relevant forum topics for each faction. Perhaps something you read will make you change your mind... ;)
The Dungeon is, to put it mildly, not the weakest faction in Heroes.
It still makes sense to first determine, before discussing each of the factions, the dependence of the hero's development on the size of the map and the opposing faction.