Heavy weapons are not simply heavy, but they often require forethought to use properly. When planning to use one, be sure to check if you're strong enough, if the weapon doesn't get a major accuracy bonus when used from a kneeling position, if it requires offhand to be free (a red '2' in Inventory) and how punishing are TU costs of its attacks. Also it is generally unadvised to fire them in melee.
A study of how these are built seems crucial to making our own heavy weapons.