Priests are terrible vs. normal units, but to compensate for this, they cost the lowest pop out of all the heroes and are generally useful, since they heal.
Hersirs do quite well against normal units, but their high food cost keeps them in check early game, and their high pop cost keeps them in check in late game.
Greek heroes are incredibly strong against any units, but they are very expensive, cost favor, and can only be created in limited numbers.
Jarls are far from raiding cavalries; think of them as Teuton Paladins from AoK (all upgrades except speed). They are only as fast as infantry, around the same speed as an ulfsark, but have an amazing amount of HP, attack, and armor. Their bonus against MUs make them even better. They are better than hippikons in combat, but are significantly slower.