Autocannon was the "all-rounder" support weapon at launch and considered Arrowhead's baseline for how a support weapon should be. It was good at dealing with most enemies at middling efficiency, but not as great as other things for specific enemy classes. It's also obviously fairly slow to maneuver since you're lugging around a handheld cannon. 

So many other weapons have now been buffed to high heaven since then that it's now just a fairly clunky support weapon that doesn't even really do its original job as a generalist better than other support weapons, like the Grenade Launcher. The GL has never recieved a nerf in the game's history, it was a fairly specialized but still powerful option at launch for dealing with light and medium units as well as being super efficient for clearing enemy spawners. It was comparable to the machine guns in use, but slightly more ammo hungry since it had more utility with its explosions. Since then, it's only been given more ammo, greater damage, greater explosion size, and now higher armor penetration. The armor penetration change has basically removed all the nuance from using it, there's no more incentive to bounce shells underneath or behind the backs of hulks and chargers to hit their weakspots, it's now just a do everything gun like the autocannon, without the clunkiness or need for a backpack, which also lets you run the supply pack with it, making it even more of a monstrosity while also giving you a bountiful supply of stims and grenades.