Wolf has 2 attacks and is excellent in melee combat.
Arbalester shoots without any penalties.
But I didn't vote for the Berserker – its lack of control makes it useless :p.
The Dwarf is strong but slow.
The Orc is versatile, but clearly weaker than the Centaur.
The Hobbit's low health and attack stat let it down.
In general, as with the situation with castles, some parameters are compensated by others, or by the number of units. It's all about balance.