I heard that the units in the companion games are more varied than the units in the base game. Can anybody explain how? Like a troll is different from a Rider of Rohan? How?

Different unit types have different characteristics. Some, such as trolls and Mumakil, have a higher combat strength. Each unit type also has one or more "favored" terrain types that give them an advantage in battle in that terrain. In addition, most unit types have a special ability that can be activated in combat to either inflict more damage on the enemy or reduce damage to themselves.