Warmachine: What makes a unit good?

The title of this article is a very heavy question. Everyone has opinions on what units are good, bad, or their favorites. I tend to look at units with a list of qualities in mind.

Cost/Unit Size/Survivable
-Is the model count low or high vs the cost?
-Is it a multi-wound unit?
-Are the arm/def above or below average?
Role
-What can the unit deal with, such as other weak infantry, heavy arm/def infantry, light/heavy jacks?
-Do I get any special abilities/rules/advantages that help me control the flow of the game?
-Advance Deploy, Stealth, Pathfinder, Magic Weapon, Tough.
Lacking
-Can other units/models fill this role better or offer things that my army lacks?


So for an example I willl break down Khador WinterGuard
Cost/Unit Size/Survivable
Winter Guard are cheap and a large unit. Their arm/def isn't anything special but you make up for it in bodies.
Role
Winter Guard are very flexible in their roles thanks to CRA. They can threaten light and hard targets but can't really dent heavy jacks/beasts that well.
Lacking
While they are all around a solid unit Winter Guard do not have anything that special to add to your army without unit attachments/weapon attachments. You might find that they are bland.