Oohhh ... the myrmidon! This puppy packs a serious punch wherever it goes. The ability to hold five weapons along with eight grenades AND the mortar gun makes the heavy armour a force to be reckoned with.
It is the only armour that can use the mortar gun, heh, and for good reasons. In addition to all of this, the amount of damage and punishment it can withstand is immense compared to the light and even medium armour. By the same token, with this new found defense you've just lost most of your maneuverability.
If you pair the myrmidon up, mortar in hand with a quicker armour with a targeting laser you've got yourself an instant cannon fodder with aim! With the discovery of skiing (the ability to tap the space bar repeatedly while running down a slope and using your jetpack at the end to fly, literally across the map) the days of using an HPC to get the myrmidon around have all but been lost.
This in mind, in the hands of an experienced tribesman few can match the power of a heavy.