I could imagine that a heavy soldier, medic, and engineer in a fortified position with super heavy weapons would be terrifying. I'm gonna give this situation a nickname. "The bullet blender." Walk into it and be torn apart. Has anyone seen the video where they plant that bomb on the reactor? Heavies would be superior in that situation. To storm the objective to take it back. Or to defend it. Most superior to defend it.
I think that people that are a little too serious about the lights are thinking that the heavies are gonna be like boomers in gears of war. They are gonna take cover until the weapon overheats or the reload happens. And they will have some advantage. Heavies can take cover too. Heavies can most like turn and acquire their targets as fast as the lights too. Add to that higher damage threshold and higher damage output AND higher ammo capacity. Have fun climbing up that ledge while I pierce your spine. Have some more bullets.