There are many questions about humans.

1. Does Haven win against everyone?
2. In what cases is the "Unstoppable Force" special ability needed?
3. Which upgrade is better: crossbowmen or archers?
4. Can rebellious troops be retrained in version 3.1?
Those are all the questions.

1. No, even with a lot of trained paladins, many can compete with him.
2. If you're playing against melee units through a thick stack, but I still wouldn't recommend leveling it up.
3. Archers are more common, but there's also a tactic with crossbowmen, for example, using teleport.
4. There's no difference, because human troops are always retrained into non-upgraded units, and it doesn't matter whether you're retraining an upgraded unit or not. And there, upgraded units can be retrained into any troops if there's a corresponding building.